The extracellular acid lipase produced by Penicillium roqueforti strain 1173 was purified by precipitation with 50 % ethanol, DEAE Sephadex A 50 ion exchange chromatography and 2 successive treatments with SP Sephadex C 50. An ~ 400-fold purification with a 35 % yield was obtained. The enzyme was active from pH 1·7–10·5, with an optimum activity pH of 6·5 at 20 °C and 6·0 at 30 °C. A second minor optimum appeared close to pH 2·5 and there was a minimum activity at pH 3·6. In the presence of 0·1 M-CaCl2 at 30 °C, the optimum was at pH 5·5, also the pH value where inhibition by CaCl2 was least (9%). The pH stability curve at 30 °C showed no discontinuities and had a maximum between pH 3·7 and 6·0. At pH 6·5 activity at 5 °C represented 37 % of the maximum value, which was reached at 35–40 °C and the apparent activation energy was about 5·42 kcal mol-1. Between pH 4·0 and 7·0, the ratio of hydrolysis velocity at 30 °C compared to that at 20 °C ranged from 2·1 to 1·2. At pH 6·0, the enzyme began to be inactivated at temperatures above 35 °C with decimal destruction times (D values) of 150, 29 and 4 min for respective temperatures of 40, 45 and 50 °C. In terms of substrate specificity the maximum activity was obtained with tricaproin whilst with tributyrin, tricaprylin, butter oil and triolein relative velocities were 22, 6·7, 3·0 and 1·5% respectively of that for tricaproin whilst Tween 20 and triacetin were 2·5 and 2% respectively.