Santa Barbara Bancorp, the $1.3-billion-in-assets holding company for Santa Barbara Bank & Trust, announced a definitive agreement to acquire Citizens State Bank of Santa Paula, which has assets of $86.4 million, in a merger valued at $16.17 million. The deal is expected to close at the end of the third quarter of 1997, subject to approval by regulators and Citizens shareholders. Citizens has three offices in the Santa Clara River Valley, two in Santa Paula and one in Fillmore. They will remain open as offices of Santa Barbara Bank. The offices will boost the bank's total to 24, with seven in Ventura County and 17 in Santa Barbara County.