Twelve styrene-utilizing bacteria were isolated from a biofilter used for treating gaseous styrene. A gramnegative strain had a high styrene-degrading activity and was identified as Pseudomonas putida SN1 by 16S rDNA analysis. The styrene degradation in SN1 was regarded to start with a monooxygenase enzyme which converted styrene to styrene oxide, a potentially important chiral building block in organic synthesis. SN1 could grow on styrene and styrene oxide, but not on benzene and toluene. The styrene degradation activity in SN1 was induced when incubated with styrene, and the induction was not inhibited by the presence of readily usable carbon sources such as glucose and citrate. The optimal activity was shown at pH 7.0 and 30 ℃ and estimated as 170 unit/g cell.