The Center City law firm Fox Rothschild said it would merge with the Minneapolis-based Oppenheimer Wolff & Donnelly in January, bringing the number of lawyers at the Philadelphia firm to more than 700.

Fox Rothschild has been on a steady growth trajectory since early 2014, when the number of lawyers stood at 530. Its strategy of focusing on a large and diverse group of clients, many of them privately held businesses, has helped to insulate it from the shocks to the legal market in 2008 and 2009 during the recession.

"This is one of those mergers that is driven by all of the good reasons," said Mark Silow, Fox Rothschild's managing partner.

"Their practice is growing and needed more depth," he said of Wolff & Donnelly. "Our practice is growing, and we have clients that need more geography and national scope. "

All of Oppenheimer's 82 lawyers will join Fox when the deal closes Jan. 4. The Oppenheimer firm was founded in 1886 and is known for commercial and health-care litigation, financial services, real estate, and other practices.

Nationally, law firms have been merging at a record pace, according to Altman Weil, the Newtown Square legal consulting firm. The firm said that through the third quarter of 2015, there had been 66 law firm combinations in the United States, the fastest pace of mergers and acquisitions since the firm began tracking such deals in 2006.

"Large and midsize law firms continue to cherry-pick desirable small firms based on practice specialty or geography," Altman Weil analyst Eric Seeger said. "We're also seeing same-city, or same-state, tie-ups between smaller firms that are joining forces in an attempt to strengthen their relative market positions."

In April, Cozen O'Connor, another Center City law firm, announced that it would absorb the 60-lawyer firm of Meckler Bulger Tilson in Chicago. Earlier in the year, Morgan Lewis & Bockius L.L.P. announced a merger with Singapore-based Stamford Law Corp., a deal that brought an additional 80 lawyers to Morgan. In late 2014, Morgan acquired much of the Boston-based law firm of Bingham McCutchen, taking on 500 additional lawyers and 250 staffers.

Silow said that absorbing regional firms in robust urban markets has tended to boost business for Fox Rothschild.

"We have an enormous client base, and we have just proven that when we are in vibrant metro areas, our existing clients just seem to generate work in those markets," Silow said.