Just weeks after it moved its headquarters from Pennsylvania to Florida to take advantage of the state’s growth opportunities, F.N.B. Corp. said Thursday that it would more than double its Pennsylvania business by acquiring Promistar Financial Corp. of Johnstown.

The company, now based in Naples, Fla., said it would pay $427 million in stock for Promistar, which has $2.4 billion of assets and 72 branches in 12 southwestern Pennsylvania counties. The deal would give F.N.B. more than $6.5 billion of assets, $5.2 billion of deposits, and 168 branches in southwestern Florida, western Pennsylvania, and northeastern Ohio.

Though F.N.B. has recently focused its efforts on faster-growing Florida markets, Gary L. Tice, its president and chief executive officer, said this deal was just too good to pass up. The acquisition would give F.N.B. access to 1.4 million additional households in Pennsylvania and create significant opportunities for it to cross-sell investment, insurance, and other fee-based products.

“From the Florida standpoint, the opportunity for growth is greater than it is in Pennsylvania,” Mr. Tice said. “If the opportunity had not been proposed, we wouldn’t have expanded in Pennsylvania.”

F.N.B. is the largest banking company headquartered in Florida. Upon the deal’s completion, it would merge Promistar into its First National Bank of Pennsylvania, which then would have the eighth-largest deposit share in the state.

Though F.N.B.’s Pennsylvania and Ohio banks cannot rival its Florida banks’ growth potential, they remain a crucial element of the company’s strategy to expand in Florida, Mr. Tice said. First National Bank of Pennsylvania “generates substantial cash flow, which can be used to fund our growing presence in Florida,” he said.

The two companies had been in discussions since the beginning of the year, and they wanted to announce a deal while they could still use pooling-of-interests accounting, he said. (Merging companies will no longer be able to initiate a pooling deal after June 30.)

John H. Anderson, Promistar’s chairman and chief executive officer, said his company wanted to team up with a larger partner because revenue growth opportunities are limited in southwestern Pennsylvania. Achieving growth in noninterest income was particularly challenging because the company could not generate enough volume to meet its profit goals, he said.

“Strategically, the deal is about revenue growth,” said Mr. Anderson, who would leave the company after the deal is closed. “Given the fact that our markets are not growth markets, we’ve been really stymied by just getting insurance, annuities, and mutual fund sales to a level that is meaningful.”

F.N.B. said it expects to increase Promistar’s fee revenues to a level comparable to its own. Promistar currently generates 17% of its income from fees; 32% of F.N.B.’s net is noninterest income.

Robert E. Kafafian, a consultant at Tucker Anthony Sutro Capital Markets in Lancaster, Pa., said that he was somewhat surprised that F.N.B. turned its sights back to the slow-growth markets of Pennsylvania but that the deal makes a lot of sense.

“If you want to get critical mass in a region, what better way than to get two good-size institutions and put them together,” he said. “This makes them a formidable presence.”

Promistar stockholders would get 0.926 share of F.N.B. for each of their shares. That would work out to about two times Promistar’s book value and about 15.9 times its expected earnings this year, in line with other recent deals. Promistar’s shares were trading at $23.47 late Thursday, up 18.8% from Wednesday’s closing price. F.N.B.’s stock was trading at $26.06, down 0.68%.

The deal, which is expected to close in the first quarter, would take somewhat longer than a typical transaction, Mr. Tice said in a conference call to investors and analysts. It is somewhat complicated since Promistar would be merged into the much smaller First National Bank of Pennsylvania, which has $1.5 billion of assets, he said.

“We want to be sure we study everything because the risk we have is losing customers,” he said.

F.N.B. said it would take a one-time charge of $41 million to cover severance agreements, professional fees, a restructuring of the investment portfolio, and operational issues.

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