501-505) as adapted by Garcia et al. (1978, Arch. Biochem. Biophys. 188: 315-322). The bugs used in our experiments consumed 160 to 200 mg of blood. Only 15% of the insects did not feed fully and were discarded. The number of bugs dissected at different intervals after infection and the number of parasites per bug are summarized in Table I. The caseinolytic activity in the crude gut preparations was found to be totally inhibited by pepstatin in all the experiments. But as can be seen, there was a high level of gut infection in both test and control groups. There was no significant difference between the number of flagellates in these two groups or in the proportion of trypomastigotes to epimastigote. The maximum numbers of trypanosomes found in a bug were 11,000 epimastigotes and 10,000 trypomastigotes for the pepstatin group, and 14,000 and 9,000, respectively, for the control group. No correlation was found between protein digestion and the degree or type of gut infection. Apparently the multiplication and differentiation of T. cruzi does not depend on activity of the R. prolixus proteinase that was inhibited. This study was supported by a grant from the Pan American Health Organization. i these two groups or in the pro-