National Penn Bancshares Inc. and Christiana Bank & Trust Co. said today that Christiana's stockholders approved the company's acquisition by National Penn for about $56.5 million.

Christiana, of Greenville, Del., will retain its name and operate as a National Penn subsidiary.

National Penn, of Boyertown, said the purchase is expected to diversify its revenue base, providing a presence in Delaware.

Christiana stockholders may elect to exchange each of their shares of Christiana common stock for either $37.69 in cash or 2.241 shares of National Penn common stock.

The deal, approved by federal and Delaware regulators, is expected to be completed on Jan. 4.

National Penn has 81 banking offices in Pennsylvania and assets of $5.8 billion. Christiana has 58 employees and assets of $164 million. It has two banking offices.

National Penn shares were down 26 cents to $15.18 in Nasdaq trading early this afternoon; Christiana shares had not traded since yesterday, when they closed at $35.50.