Among the 452 bacterial isolates obtained from the soil samples (PO1-PO5) and waste water (PO6-PO12) associated with the palm oil refinery, mangrove sediments from the south of Thailand (MS1-MS18), and soil contaminated with ULO (US1-US18), only 48 bacterial isolates showed positive results with the drop-collapse method. Microbial growth in the production medium was used as a parameter during the primary screening of the biosurfactant-producing bacteria. After centrifugation if the fermentation broth, only a few strains showed a significant amount of biomass in the form of pellets, which indicated their abilities to grow in the production medium. Table 1 shows the Gram staining results, growth, emulsification activity, and surface activity of selected bacterial isolates. Of the selected bacterial isolates, 78% (37/48) were Gram-negative. It has previously been reported that most bacteria isolated from sites with a history of contamination by hydrocarbon oil or its by-products and other immiscible substrate are Gram-negative due to the presence of outer membranes which act as biosurfactants (Batista et al. 2006; Ruggeri et al. 2009; Saimmai et al. 2012a, c). Howwever, in our study, the highest biosurfactants activity was identified in a Gram-positive bacterium (PO5) isolated from a soil contaminated by a palm oil refinery in Phang Nga Province, with a drop-collapse result of 4.0 mm. Interestingly, this strain did not show high emulsification activity (41.26%), but it could produce biosurfactants which exhibitied the lowest surface tansion value (39.5 mN/m) among those tested in our study. One explanation for growth and also the type of hydrocarbon utilized in the emulsification activity test. When the medium is a single phase type (water-soluble carbon source), with no requirement for an emulsifier to make an insoluble substrate more accessible, there is no relation between surface tension reduction and emulsification activity (Saimmai et al. 2012c).