Sun Life Financial Inc. today announced it has agreed to sell its U.S. retirement plan administration business to Hartford Financial Services for an undisclosed price.

The deal is expected to close in the first quarter of 2008.

Sun Life Retirement Services (U.S.) Inc. administers 401(k) defined-contribution retirement plans, serving about 6,000 plan sponsors and 465,000 participants in the United States.

“After a thorough review, we determined that the best course of action is to focus on parts of the U.S. retirement and wealth market where we have or can achieve competitive advantage, scale and a market leadership position,” Bob Salipante, president of Sun Life Financial U.S., said Thursday in a release.

“Among a number of important strengths, I am delighted the depth of talent and experience of RSI’s staff was a key factor in The Hartford’s decision to acquire the business.”

Toronto-based Sun Life Financial says it has a significant presence in asset management through subsidiary Massachusetts Financial Services Co., which has distribution relationships with RSI and The Hartford.