SYNOPSISThe insectivorous bugZelus leucogrammus(Perty, 1834) (Hemiptera, Reduviidae) was found to harbor several species of trypanosomatids of the generaLeptomonas, Crithidia, HerpetomonasandBlastocrithidia. However, laboratory bred nymphs given sterile food were infected only byBlastocrithidia, the other flagellates appearing in their intestinal tract when they were fed on other insects.TheBlastocrithidiawas thought to belong to a single species and to be a parasite proper ofZ. leucogrammus(and possibly other Hemiptera), producing luxuriant infections in the intestinal tract of the bugs captured in nature and spreading easily among isolated populations through cannibalism and ingestion of cysts; it was also abundant in the genital tract of gravid females but transovarian transmission was not proved. ThisBlastocrithidiaexperimentally infectedTriatoma infestansand is very close toB. triatomaeCerisola, del Prado, Rohwedder and Bozzini, 1971.The other trypanosomatids, never numerous in naturally infectedZ. leucogrammus, are acquired by the bug from many different insects, chiefly Diptera, on which it was seen to feed.Herpetomonas muscarumandC. luciliae, the common trypanosomatids among several muscoid Diptera, were isolated both from flies and from the bugs fed on them.