Trypsin-like, chymotrypsin-like, aminopeptidase and carboxypeptidase A and B were detected in the midgut of adult female Anastrepha suspensa (Loew). Based on specific substrates and inhibitors, endoproteinases were demonstrated to be serine proteinases in which trypsin-like enzyme seemed to be the major digestive enzyme. These proteinases had alkaline pH optima, pH 7.5–9. All determined proteinase activities in the midgut were reduced to different degrees in the first few days after newly emerged flies were treated for 24 h with 0.2 and 0.5% sodium tetraborate. The 0.5% concentration caused greater reductions. These reductions in proteinase activities were not the result of direct enzyme inhibition.