The pathology and attachment of Blastocrithidia triatomae in two regions of the midgut of the reduviid bug Triatoma infestans (the stomach and the small intestine) were investigated by electron microscopy. In both regions the extracellular-membrane layers and the apical microvilli were often reduced, and some cells of the intestinal wall were vacuolated. In the stomach two types of attachment occurred: Flagella with intraflagellar swellings lay over and between the apices of the microvilli of the stomach cells, and in microvilli-free regions the cell body of B. triatomae seemed to be anchored to the host cell by corrugations of both cells in the attachment zone, a hitherto unknown mode of attachment of trypanosomatids in the intestinal tract. In the small intestine flagellar expansions held several microvilli, and rarely an unaltered flagellum seemed to be anchored between the microvilli. In both midgut regions, the flagellum of some epimastigotes was inserted into the apex of an intestinal cell.