The optimal conditions for the activity of the principal midgut digestive enzymes, e.g., amylase, protease and lipase have been ascertained in the final instar larva and adult of T. virginia. Although the peak activity of the enzymes is at similar pH, buffer concentration and temperature, a wide discrepancy occurs among the other parameters, particularly the substrate and enzyme concentrations and the incubation period. Midgut digestive enzyme activity is higher in the final instar larva than in the adult.