Four Thermus strains produced lipolytic activity when grown in liquid medium for 30 h at 70degreesC. The highest total lipase/esterase activity (57 U l(-1)) was in Thermus aquaticus YT-1, followed by Thermus thermophilus HB27 and HB8 (33 and 25 U l(-1), respectively), and finally by Thermus sp. (16 U l(-1)). Extra-cellular activity was detected in T. aquaticus YT-1 and T. thermophilus HB27 (33 and 17 U l(-1)). All enzymes were stable at 80. C over 30 min, and their activity towards fatty acid esters increased as substrate chain-length diminished (i.e. hydrolysis rate was up to 6-fold higher on p-nitrophenyl caproate than on laurate).