Benefits consulting firm Mercer is acquiring rival Callan Associates, doubling its investment consulting presence in the U.S. at a time when corporations are grappling with pension and other human-resource issues.

Mercer, a subsidiary of Marsh & McLennan Cos. (MMC), didn't disclose the sum it is paying for privately held Callan when it announced the deal Tuesday. The acquisition is expected to close by the end of this quarter.

Callan and Mercer have been fierce competitors for consulting clients, and since most of their customers just hire one firm, they will have little overlap, executives said during a telephone interview.

Mercer, with 18,000 employees spread throughout the world, has about 200 of its 950 global investment consulting staff based in the U.S. Callan focuses solely on the U.S., and employs more than 170 people. By combining, Mercer will nearly double its U.S. presence and Callan will gain access to international client services.

"This is very much a strategic merger," said Jeff Schutes, Mercer's head of U.S. investment consulting. Companies are facing economic conditions that "are more challenging than ever, and our two organizations combined will have the resources to assist them."

Both Mercer and Callan have a strong focus on retirement benefits consulting, a business that includes advising on pension-plan issues and selecting money managers. Callan also provides other services, such as back-office services to other consultants and educational workshops for corporations and public pensions.

One of the primary services the pair will specialize in is analyzing how companies stack up against peers on benefit issues. Callan maintains a database that will roughly double in size once the two organizations unite, giving them a broader perspective on what practices corporations and public pensions are engaging in, said Gregory Allen, president of Callan.

The larger size of the combined entity will also make it easier to hire skilled staff, Allen said.

"The consulting industry has always competed with the asset-management industry in hiring, so by becoming larger and having more resources" the firm will be better able to hire talent, Allen said.

-By Lynn Cowan, Dow Jones Newswires; 301-270-0323; lynn.cowan@dowjones.com