E-64, a specific inhibitor of cysteine proteinases, was incorporated into artificial seeds at low levels (0.01–0.25% by weight). It prolonged developmental time and increased mortality of the larval cowpea weevil,Callosobruchus maculatus(F.), in direct proportion to its concentration in the artificial seeds. The fecundity of females emerging from the artificial seeds was significantly decreased by E-64 concentrations of 0.06% and higher. These observations are compatible with the hypothesis that the midgut cysteine proteinase inC. maculatusis essential for normal growth and development.