The fatty acid specificity and pH dependence of triacylglycerol hydrolysis by the Rhizopus delemar lipase acylbinding site mutant Val206Thr+Phe95Asp (Val, valine; Thr, threonine; Phe, phenylalanine; Asp, aspartic acid) were characterized. The activity of the double mutant prolipase was reduced by as much as 10-fold, compared to the wild-type prolipase. However, the fatty acid specificity profile of the enzyme was markedly sharpened and was dependent on the pH of the substrate emulsion. At neutral pH, strong preference (10-fold or greater) for hydrolysis of triacylglycerols of medium-chainlength fatty acids (C8:0 to C14:0) was displayed by the variant prolipase, with no hydrolysis of triacylglycerols of short-chain fatty acids (C4:0 to C6:0) and little activity manifested toward fatty acids with 16 or more carbons. At acidic pH values, the fatty acid selectivity profile of the double mutant prolipase expanded to include short-chain triacylglycerols (C4:0, C6:0). When assayed against a triacylglycerol mixture of tributyrin, tricaprylin and triolein, the Val206Thr+Phe95Asp prolipase displayed a high selectivity for caprylic acid and released this fatty acid at least 25-fold more efficiently than the others present in the substrate mixture. When presented a mixture of nine fatty acid methyl esters, the wild-type prolipase showed a broad substrate specificity profile, hydrolyzing the various methyl esters to a similar extent. Contrastingly, the double mutant prolipase displayed a narrowed substrate specificity profile, hydrolyzing caprylic methyl ester at nearly wild-type levels, while its activity against the other methyl esters examined was 2.5- to 5-fold lower then that observed for the wild-type enzyme.