Extracellular poly(3-hydroxybutyrate) depolymerase was purified from the culture medium of Peudomonas lemoignei and separated into four isozymes (A 1, A 2, B 1 and B 2) by CM-Sepharose CL-6B chromatography. The molecular weights of A 1 and A 2 and those of B 1 and B 2 were estimated to be 54000 and 58000, respectively, by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The isoelectric points of A 1, A 2, B 1 and B 2 were found to be approximately pH 9.7 10.0, 10.0 and 10.6, respectively, by isoelectric focusing. All four enzymes hydrolyzed poly(3-hydroxybutyrate) and oligomeric esters of d-(−)-3-hydroxybutyrate, but showed no activity toward the dimeric ester. Analysis of hydrolytic products of the oligomeric esters with A 1 and B 2 suggested that the enzymes hydrolyzed mainly the second and third ester bonds from the free hydroxy terminus at different frequencies, depending upon the chain length of the substrates.