An aryl-phosphate ester (APE)-degrading bacterium was isolated from the leachate of a sea-based waste disposal site. The isolated APE-degrading bacterial strain YS-57 grew well in a medium containing glucose and NaCl, and degraded two types of APE: tricresyl phosphate and triphenyl phosphate. The optimal temperature, pH, and NaCl concentration for the growth of strain YS-57 were 30degreesC, 7.0, and 1.0%, respectively. Strain YS-57 grew at an APE concentration of 25 mg/l without being inhibited. APEs were degraded by the supernatant of the medium in which strain YS-57 was cultured, suggesting that the APE-degrading enzyme was released into the extracellular space in the logarithmic growth phase. The 16S rDNA sequence of strain YS-57 showed 95.6% similarity to that of Roseobacter gallaeciensis and the morphological properties were also comparable. Consequently, strain YS-57 was closely related to the genus Roseobacter.