Activity of acidic proteinase in the midgut of larval Parasarcophaga surcoufi was investigated and partially characterized. Larval midgut extract showed a moderate acidic pH (pH 4) optimum for hydrolysis of haemoglobin. The proteolytic activity of the larval midgut was estimated by hydrolysis of haemoglobin, and was found to be inhibited by pepstatin (aspartic proteinase inhibitor), dithiothreitol (DTT) and mercaptoethanol; while it was not inhibited by the soybean trypsin inhibitor (STI) and EDTA. These characteristics may imply that cathepsin D-like proteinase is an effective acidic proteinase present in the Parasarcophaga surcoufi larval midgut.