Six months after PacWest Bancorp first offered to acquire First California Financial Group Inc., the bank holding companies have agreed to a deal, the two parties announced late Tuesday.

Century City’s PacWest, the parent of Pacific Western Bank, will pay $231 million for First California in Westlake Village. At $8 a share, the all-stock deal represents a 19 percent premium over First California’s Nov. 6 closing price.

The deal, expected to close in the first quarter, gives PacWest pro forma assets of $7.5 billion, making it the fourth largest bank headquartered in Los Angeles County. First California’s subsidiary, First California Bank, will be merged into Pacific Western Bank, giving it 81 branches throughout the state.

“We are very pleased to have reached this agreement with First California,” Matt Wagner, PacWest’s chief executive and chairman, said in a statement. “The merger of these two organizations will create a valuable franchise for the stockholders of both companies.”

In May, First California announced that PacWest had made an “unsolicited and highly conditional offer” to acquire the company. First California did not accept the offer and instead said it would consider other strategic alternatives, prompting criticism from several large shareholders, including Castine Capital Management in Boston.

In August, First California’s board hired advisers to consider a possible sale.

In Nasdaq trading Wednesday, First California shares closed up 90 cents, or 12.5 percent, to $7.60. PacWest shares closed down 12 cents, or less than 1 percent, to $23.07.