SCHEDULE 13G

Amendment No. 4 
EXFO Electro-Optical Engineering Incorporated 
Subordinate Voting Common Stock 
Cusip #302043104 


Cusip #302043104 
Item 1:	Reporting Person - FMR Corp.
Item 4:	Delaware
Item 5:	68,100 
Item 6:	0 
Item 7:	4,583,100 
Item 8:	0 
Item 9:	4,583,100 
Item 11:	15.000% 
Item 12:	    HC


Cusip #302043104  
Item 1:	Reporting Person - Edward C. Johnson 3d 
Item 4:	United States of America
Item 5:	0 
Item 6:	0 
Item 7:	4,583,100 
Item 8:	0 
Item 9:	4,583,100 
Item 11:	15.000% 
Item 12:	IN 


Cusip #302043104 
Item 1:	Reporting Person - Abigail P. Johnson 
Item 4:	United States of America
Item 5:	0 
Item 6:	0 
Item 7:	4,583,100 
Item 8:	0 
Item 9:	4,583,100 
Item 11:	15.000% 
Item 12:	    IN



	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)




Item 1(a).	Name of Issuer:	

		EXFO Electro-Optical Engineering 
Incorporated 

Item 1(b).	Name of Issuer's Principal Executive Offices:

		465 Godin Avenue  
		Vanier, Quebec, Canada  G1M 2K2
		  

Item 2(a).	Name of Person Filing: 

		FMR Corp.

Item 2(b).	Address or Principal Business Office or, if None, 
Residence:

		82 Devonshire Street, Boston, 
Massachusetts  02109

Item 2(c).	Citizenship:

		Not applicable

Item 2(d).	Title of Class of Securities:

		Subordinate Voting Common Stock 

Item 2(e).	CUSIP Number:  

		302043104 

Item 3.	This statement is filed pursuant to Rule 13d-1(b) or 13d-2(b) 
and the person filing, FMR Corp., is a parent holding company 
in accordance with Section 240.13d-1(b)(ii)(G).  (Note:  See 
Item 7).

Item 4.	Ownership

	(a)	Amount Beneficially Owned:	4,583,100 

	(b)	Percent of Class:	15.000% 

	(c)	Number of shares as to which such 
person has:  

	(i)	sole power to vote or to direct 
the vote:	68,100 

	(ii)	shared power to vote or to 
direct the vote:	0 

	(iii)	sole power to dispose or to 
direct the disposition of:	4,583,100 

	(iv)	shared power to dispose or to 
direct the disposition of:	0 


Item 5.	Ownership of Five Percent or Less of a Class.

	Not applicable.

Item 6.	Ownership of More than Five Percent on Behalf of Another 
Person.

	Various persons have the right to receive or the power to 
direct the receipt of dividends from, or the proceeds from the 
sale of, the Subordinate Voting Common Stock of EXFO 
Electro-Optical Engineering Incorporated.  The interest of one 
person, Canadian Growth Company Fund, an investment 
company registered under the Investment Company Act of 
1940, in the Subordinate Voting Common Stock of EXFO 
Electro-Optical Engineering Incorporated, amounted to 
1,898,000 shares or 6.212% of the total outstanding 
Subordinate Voting Common Stock at December 31, 2004. 
The interest of one person, FICL Canadian Asset Alloc-cad, 
an investment company registered under the Investment 
Company Act of 1940, in the Subordinate Voting Common 
Stock of EXFO Electro-Optical Engineering Incorporated, 
amounted to 2,500,000 shares or 8.182% of the total 
outstanding Subordinate Voting Common Stock at December 
31, 2004. 

Item 7.	Identification and Classification of the Subsidiary Which 
Acquired the Security Being Reported on By the Parent 
Holding Company.

	See attached Exhibit(s) A, B. 

Item 8.	Identification and Classification of Members of 
the Group.

	Not Applicable. See attached Exhibit A. 

Item 9.	Notice of Dissolution of Group.

	Not applicable.

Item 10.	Certification.

	By signing below I certify that, to the best of my knowledge 
and belief, the securities referred to above were acquired in the 
ordinary course of business and were not acquired for the 
purpose of and do not have the effect of changing or 
influencing the control of the issuer of such securities and 
were not acquired in connection with or as a participant in any 
transaction having such purpose or effect.

Signature

	After reasonable inquiry and to the best of my knowledge and 
belief, I certify that the information set forth in this Schedule 
13G in connection with FMR Corp.'s beneficial ownership of 
the Subordinate Voting Common Stock of EXFO Electro-
Optical Engineering Incorporated at December 31, 2004 is 
true, complete and correct. 

February 14, 2005 			
Date

/s/Eric D. Roiter				
Signature

Eric D. Roiter				
Duly authorized under Power of Attorney
dated December 30, 1997 by and on behalf
of FMR Corp. and its direct and indirect
subsidiaries


	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)



	Pursuant to the instructions in Item 7 of Schedule 
13G, Fidelity Management & Research Company 
("Fidelity"), 82 Devonshire Street, Boston, Massachusetts 
02109, a wholly-owned subsidiary of FMR Corp. and an 
investment adviser registered under Section 203 of the 
Investment Advisers Act of 1940, is the beneficial owner of 
4,583,100 shares or 15.000% of the Subordinate Voting 
Common Stock outstanding of EXFO Electro-Optical 
Engineering Incorporated  ("the Company") as a result of 
acting as investment adviser to various investment companies 
registered under Section 8 of the Investment Company Act of 
1940.

	The ownership of one investment company, Canadian 
Growth Company Fund, amounted to 1,898,000 shares or 
6.212% of the Subordinate Voting Common Stock 
outstanding. Canadian Growth Company Fund has its 
principal business office at 82 Devonshire Street, Boston, 
Massachusetts 02109.

	The ownership of one investment company, FICL 
Canadian Asset Alloc-cad, amounted to 2,500,000 shares or 
8.182% of the Subordinate Voting Common Stock 
outstanding. FICL Canadian Asset Alloc-cad has its principal 
business office at 82 Devonshire Street, Boston, 
Massachusetts 02109.

	Edward C. Johnson 3d, FMR Corp., through its 
control of Fidelity, and the funds each has sole power to 
dispose of the 4,583,100 shares owned by the Funds.

	Neither FMR Corp. nor Edward C. Johnson 3d, 
Chairman of FMR Corp., has the sole power to vote or direct 
the voting of the shares owned directly by the Fidelity Funds, 
which power resides with the Funds' Boards of Trustees.  
Fidelity carries out the voting of the shares under written 
guidelines established by the Funds' Boards of Trustees.

	Members of the Edward C. Johnson 3d family are the 
predominant owners of Class B shares of common stock of 
FMR Corp., representing approximately 49% of the voting 
power of FMR Corp.  Mr. Johnson 3d owns 12.0% and 
Abigail Johnson owns 24.5% of the aggregate outstanding 
voting stock of FMR Corp.  Mr. Johnson 3d is Chairman of 
FMR Corp. and Abigail P. Johnson is a Director of FMR 
Corp.  The Johnson family group and all other Class B 
shareholders have entered into a shareholders' voting 
agreement under which all Class B shares will be voted in 
accordance with the majority vote of Class B shares.  
Accordingly, through their ownership of voting common 
stock and the execution of the shareholders' voting 
agreement, members of the Johnson family may be deemed, 
under the Investment Company Act of 1940, to form a 
controlling group with respect to FMR Corp.

 


	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)
	RULE 13d-1(f)(1)  AGREEMENT

	The undersigned persons, on February 14, 2005, agree 
and consent to the joint filing on their behalf of this Schedule 
13G in connection with their beneficial ownership of the 
Subordinate Voting Common Stock of EXFO Electro-Optical 
Engineering Incorporated at December 31, 2004.

	FMR Corp.

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Duly authorized under Power of Attorney
	dated December 30, 1997, by and on behalf
	of FMR Corp. and its direct and indirect 
	subsidiaries

	Edward C. Johnson 3d

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Duly authorized under Power of Attorney
	dated December 30, 1997, by and on behalf
	of Edward C. Johnson 3d

	Abigail P. Johnson

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Duly authorized under Power of Attorney
	dated December 30, 1997, by and on behalf
	of Abigail P. Johnson

	Fidelity Management & Research Company

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Senior V.P. and General Counsel

	Canadian Growth Company Fund

	By   /s/John H. Costello 			
	John H. Costello
	Fidelity Investments Canada Limited, as
	trustee of  Canadian Growth Company Fund

	FICL Canadian Asset Alloc-cad

	By   /s/John H. Costello 			
	John H. Costello
	Fidelity Investments Canada Limited, as
	trustee of  FICL Canadian Asset Alloc-cad