Fidelity Investments has lost hundreds of corporate customers in its defined-contribution retirement plan business over the past year, a challenge for the giant mutual fund firm in a critical segment of the industry.

Fidelity’s Workplace Investing division provided services to about 17,100 corporate defined-contribution plans as of March 31, down from 17,200 at the end of 2009 and 17,500 at June 30, 2009, according to confidential debt offering documents obtained by Reuters.

Boston-based Fidelity has ridden the rapid growth of the 401(k) retirement savings plan market since the 1980s but now faces growing competition from firms also looking for a share of the trillions of dollars up for grabs in retirement savings.

Earlier this year, Fidelity lost two big names — Apple Inc and Ford Motor Co — to competitors Charles Schwab Corp. and Affiliated Computer Services, now a unit of Xerox Corp . Fidelity added General Electric Co as a client around the same time.

Despite the recent losses, Fidelity remains by far the largest service provider to workplace retirement plans, whether measured by the number of plans or dollars in the plans, said John Meunier, principal at Cogent Research in Boston.

Fidelity oversees almost one-quarter of all 401(k) plan assets, according to a recent Cogent survey. Fidelity administered corporate plans totaling $759 billion at the end of March, up from $626 billion at the end of June 2009.

Fidelity’s greatest strength is its well-known brand name, while competitors have been undercutting its prices. “The main reason driving clients to them is their reputation and brand,” Meunier said. “They don’t score as well on costs and fees.”

Retirement plans have been a key source of assets feeding into Fidelity’s $1.5 trillion money management business.

Speculation that Fidelity’s parent company, FMR LLC, eventually might go public has increased along with the age of its chairman and chief executive, Edward “Ned” Johnson, who turned 80 last month. But the company said in a confidential debt prospectus that it currently has no such plans.

“FMR is not and does not intend to become a public reporting company,” Fidelity said in its most recent prospectus, dated June 30.

The document also gives an updated financial picture of Fidelity, which typically releases only a tiny amount of information about its results once a year.

First-quarter net income was $430 million, compared with a loss of $115 million for the same period of 2009. Revenue grew on rising markets, and further job cuts controlled its expenses.

Including non-financial businesses it partially owns, the company had 51,600 employees as of March 31, down from 52,500 at the end of 2009.