A U.S. financial services company is acquiring a large retirement plan operation from its Canadian owner.

Hartford Life, Simsbury, Conn., a unit of Hartford Financial Services Group Inc., Hartford, has agreed to acquire Sun Life Retirement Services (U.S.) from the U.S. division of Sun Life Financial Inc., Toronto, the companies say.

Sun Life Retirement Services previously was known as MFS Retirement Services.

The deal does not include the investment company parent of the retirement services unit, Massachusetts Financial Services Company.

Sun Life announced a year ago that it had hired advisors to seek “strategic alternatives” for the investment company operations. Sun Life ended up deciding to keep the investment company.

The Sun Life retirement services arm employees about 400 people. It provides recordkeeping and administrative services for defined contribution retirement plans that serve 6,000 plans with 465,000 participants with $17 billion in assets.

The price of the retirement services is not available, but Hartford says it will be keeping the retirement services unit offices in Boston and Phoenix and offering jobs to all of the unit’s employees.

Hartford and Sun Life hope to close on the deal by March 31, 2008.

“This acquisition clearly boosts our retirement plans business, helping to accelerate growth, open new markets and better serve our customers,” John Walters, president of Hartford’s U.S. wealth management group, says in a statement about the deal.