Mass production of a copolymer consisting of S-hydroxybutyrate and 3-hydroxyvalerate, P(3HB-co-3HV), from Methylobacterium organophilum NCIB 11278 was investigated by using an automatic fed-batch culture technique. P(3HB-co-3HV) was produced when valeric acid was supplied as an auxiliary carbon source with methanol as the main carbon source. Among the sir nutrients tested (NH4+, K+, Mg-2(+), SO42-, PO42- and dissolved oxygen), the synthesis of P(3HB-co-3HV) was most favored in conditions of potassium limitation. The effect on copolymer production of the ratio of valeric acid to methanol (V/M ratio) in the feed was examined. At a V/M ratio of 1/10 (1 ml valeric acid per 10 ml methanol), a copolymer content of 41% of dry cell weight and a 3-hydroxyvaleric acid (3HV) fraction in the copolymer of 14 mol% were obtained. During 78 h of cultivation under these optimized conditions, accumulation of P(3HB-co-3HV) was as high as 99 g l(-1) with a 3HV fraction of 11.0 mol%.