Bacillus stearothermophilus has been reported to produce an extracellular esterase with molecular weight of 42-47 kDa. Extracellular esterase activity in Bacillus, stearothermophilus (NCIB 13335) was found to reside in protein with a molecular weight less than 10 kDa. This small esterase was responsible for all the esterase activity observed in this strain under the conditions studied.