Biosurfactants are amphiphilic molecules with effective surface-active and biological properties applicable to replace synthetic surfactant in several industries and processes. In this study biosurfactant producing bacteria were isolated from 50 terrestrial samples collected in areas contaminated with petroleum in several districts in Bangkok, Thailand. The biosurfactant producing ability of bacteria was investigated by oil displacement agar plate and emulsification index method. A total of 42 biosurfactant producing bacteria were isolated from the samples. The best biosurfactant producing isolate that showed the highest oil displacement and emulsification activity was identified as Serratia marcescens based on microscopic and biochemical analysis.