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Trypsin production in the malaria vector Anopheles tessellatus Theobald peaks between 12 and 21 h after a blood meal. The presence of leupeptin or soybean trypsin inhibitor in a blood meal delayed the onset of maximal trypsin activity. Trypsin inhibitors in an infective blood meal increased the infectivity of Plasmodium vivax Grassi and decreased infectivity of P. falciparum Welch to An. tessellatus. The opposite effects of trypsin inhibitors on infectivity of the 2 malaria parasites were attributed to differences in the biology of the parasites within the midgut of the vector, particularly the time of ookinete formation and the requirement for activation of a chitinase.