A novel bacterial strain capable of growing in a medium containing a medium-chain-length polyhydroxyalkanoate (MCL-PHA) as the sole carbon source was isolated from a soil sample, The isolate, which was identified as Pseudomonas sp. RY-1, secreted MCL-PHA depolymerase into the culture fluid only when it was cultivated in a medium containing a MCL-PHA, such as polyhydroxyoctanoate (PBO) or polyhydroxynonanoate (PHN). The extracellular MCL-PHA depolymerase of this organism was purified to electrophoretic homogeneity. The enzyme was a tetramer with identical subunits and a total molecular mass of 115 kDa. The isoelectric point of this enzyme was estimated to be 5.9 by isoelectric focusing. The maximal activity was observed. at pH 8.5 and 35 degrees C. The enzyme was insensitive to phenylmethylsulfonyl fluoride and dithiothreitol, unlike other short-chain-length (SCL) PRA depolymerases, The K-m values for PHO and PHN were 0.86 and 1.47 mg/ml, respectively. The enzyme could not hydrolyze SCL-PHAs and p-nitropbenyl esters of fatty acids.