Proteolytic activity in whole insect extracts of the western flower thrips, Frankliniella occidentalis, was found to belong predominantly to the class of cysteine proteases. The pH optimum of the general proteolytic activity was determined to be 3.5, which is low when compared to other insects using cysteine proteases for protein digestion. The proteinaceous cysteine protease inhibitors chicken cystatin, potato cystatin and sea anemone equistatin inhibited in vitro more than 90 f the protease activity. To test in vivo the biological effect of such inhibitors on the oviposition rate of western flower thrips, recombinant potato cystatin and equistatin were fed to adult females. A gradual reduction in oviposition rate to about 45 f control was observed when reared on these PIs for a period of 5 days, with no increase in mortality. These results are discussed in the light of the application of protease inhibitors in transgenic plants to control this insect pest.