Lipase from Pseudomonas fluorescens was studied for thermostability at temperatures ranging from 100 C to 160 C. The heat treatments were in two media, and heating times necessary to inactivate 90% of the enzyme at constant temperature were extremely long even at high temperatures, e.g. 3.6 min at 140 C in nutrient broth and 2.0 min at 170 C in skim milk. The increments of temperature to reduce these heating times 90% were 37.0 C in nutrient broth and 38.9 C in skim milk. The lipase was inactivated only partly after 20 h at 20 C in 8 M urea, 6 M guanidine hydrochloride, and 1.0% sodium dodecyl sulfate. Four percent 2-mercaptoethanol showed no effect.