Poly(aspartic acid) (PAA) hydrolase was purified from Sphingomonas sp. KT-1 (JCM10459). The purified hydrolase degraded thermally synthesized PAA to oligomers. The molecular mass of PAA hydrolase was 30 kDa and the isoelectric point was 8.9. The optimum values of pH and temperature for PAA degradation were 10.0 and 40 degreesC, respectively. The investigation of the effect of inhibitors for the PAA-degrading activities has revealed that the PAA hydrolase is a serine-type hydrolase. The structural analysis of PAA-degraded products using H-1 and C-13 nuclear magnetic resonances has indicated that the purified enzyme hydrolyzes selectively the beta-amide linkage connecting with beta-aspartic acid units in PAA.