The digestive system of Ceratitis capitata was characterized during its larval development and in the insect stage. Disaccharidases against maltose and sucrose were more evident in the 2nd and 3rd day of larval development and in the adult stage, respectively. Glycosil-hydrolyses with higher specific a-galactosidasic and b-galactosidasic activities were detected in the 2nd and 3rd day of the larval stage, respectively. Specific proteolytic activities against azocasein showed an increase in the 4th and 5th day of the larval stage and in the adult stage. Specific hemoglobin activities were constant between 2nd and 6th day of the larval stage. The larvae used mainly serine proteinases, such as trypsin/chymotrypsin, and the adult insects only chymotrypsin-like enzymes in their digestive process. Two serine proteinases were separated from zymogram between the 4th and 5th day of larval development and in the adult stage. Effect of soybean trypsin inhibitor (SBTI, a serine proteinase inhibitor) on development of C. capitata was examined by bioassay. C. capitata was susceptible to SBTI which affected larval mass at ED 50 3.01%. r