Thirty-nine bacterial moderate thermophilic strains growing on triolein as sole carbon and energy source were isolated from soil samples by an enrichment culture on a mineral medium. When grown on a Tween 80 medium, most of the strains showed very high specific growth rates in the range of 1-2 h(-1) which corresponded to doubling times of 20-40 min. A high esterase activity was detected in the cell culture of all strains when assayed on soluble p-nitrophenyl caprylate (pNPC8). Strain MAS2 showed the highest esterase activity (around 40 U l(-1)). This activity was equally recovered in the culture supernatant and cell fraction. The majority of the cell esterase activity was recovered in the supernatant after high speed centrifugation, suggesting a soluble localization. Activity on pNPC16 was only recovered in the high speed centrifugation pellet, suggesting a membrane localization for the lipase activity. The best medium for pNPC8 production was a synthetic medium containing both glucose and yeast extract. The culture medium supernatant showed the highest activity on p-nitrophenyl acetate (pNPC2) than on pNPC8 and no activity an pNPC16, indicating the presence of esterase rather than lipase activity. This pNPC8 activity was stable (98%) after a 1 h treatment at 70 degrees C which renders this new esterase very attractive for biotechnological applications.