Fox Rothschild's stealth growth strategy has found another target of opportunity: Seattle.

The Philadelphia-based law firm has acquired a 39-lawyer firm there, giving it a presence in the Pacific Northwest and continuing its steady expansion across the United States. Fox Rothschild focuses on what it says are economically vibrant cities that have escaped the notice of other large national firms.

"It is a big, vital city with a great business community, and it seems to be somewhat underserved by the larger national firms," said Mark Silow, Fox Rothschild LLP's chair. "So there are more opportunities."

The merger with Riddell Williams P.S. brings the lawyer head count to about 800 — Fox Rothschild has been growing ever since the 2008-2009 recession, when it had about 500 lawyers. Riddell Williams, though small, is a well-regarded general service firm that focused on, among other practice areas, corporate, labor and employment, creditors' rights, and litigation.

Riddell Williams has been in the Seattle area for more than 100 years. The acquisition gives Fox Rothschild a ready-made package of relationships and clients that would have taken years to achieve had it gone in cold and tried to develop a legal beachhead on its own.

"We never follow the colonial approach of sending in one of our lawyers from out of town to open up a new market," Silow said. "We open offices because of the convergence of two factors: We like the market in general, and we like the people we are merging with."

Fox Rothschild has had its eye on Seattle for about a decade. Before the recession, it had lengthy merger discussions with another firm there that ended after Fox Rothschild leaders determined that the potential partner didn't match up well financially. In short, Fox Rothschild was more profitable, and that would have created problems down the road.

"It would have a destabilizing effect if people were making widely differing amounts of money," Silow said generally of the difficulties inherent in merging with a less profitable firm. "It's hard to get people excited about bringing in a group" that's generating less revenue per lawyer.

For a large law firm, Fox Rothschild has a unique business model, composed of a broad base of middle-market companies and privately held firms. In all, it has something on the order of 18,000 clients, a number that has grown 20 percent since 2008. During the recession, the law firm wasn't as exposed as others to structured-finance clients or other Wall Street business, and it was one of the few large firms that managed to make it through the downturn without layoffs. In fact, it grew steadily throughout the recession.

In addition to Seattle, in recent years Fox Rothschild has expanded in Dallas, Chicago, Minneapolis, and Denver, and now has 22 offices in 13 states and the District of Columbia. Though the firm represents a number of offshore clients with matters in the U.S. and regularly sends its lawyers overseas, Silow said it sees no need to open an office abroad.

In a move similar to its Seattle merger, Fox Rothschild acquired 82-lawyer Oppenheimer Wolff & Donnelly LLP in 2015, a Minneapolis-based firm with a long history there and well-established business contacts. The theory then, as now, is that existing clients will give the firm even more work when it opens in a new market, because those clients likely have business in those cities, too.