The molecular mass of esterases usually falls in the range of 20-160 kDa, although an esterase of 5.7 kDa from Candida lipolytica has been described. Three other enzymes smaller than 10 kDa have been reported, all of which were more thermostable than their higher molecular mass counterparts. This paper describes the purification of an extracellular esterase hydrolysing fluorescein dibutyrate from Bacillus stearothermophilus NCIMB 13335. The esterase had a molecular mass of 1.57 kDa when analysed by SDS-PAGE, gel filtration and MALDI-TOF spectrometry. This enzyme retained more than 90% of its activity after incubation at 90°C for 2 h.