Phospholipase D, from cabbage, is active in reverse micelles formed from its substrate phosphatidylcholine and Triton X-100 in diethyl ether. The activity is optimum at w(0)=12.5. The increase of the molar ratio of Triton X-100/substrate from 1:4 to 2:1 results in an activity decrease by 25%. At 136 mM Triton X-100 the K-M value in reverse micelles is 136 mM, whereas it is 0.40 mM in the aqueous system containing SDS.