To investigate the cellular basis of our recent observation that the trypanosome Blastocrithidia triatomae disturbs excretion in Triatoma infestans, we compared the morphology and ultrastructure of Malpighian tubules in infected and noninfected bugs. Tubules of bugs infected 3-4 months before dissection did not show any morphological alterations in either the upper or lower regions. In fifth instars with a long lasting infection (9-10 months) and a correspondingly retarded larval development, the tracheal system supplying the Malpighian tubules was reduced and alterations were conspicuous in the upper region, where the tubules were slightly widened, sometimes having parts with conspicuous swellings. The cells were filled with white concretions and a strong autofluorescence was evident by fluorescence microscopy. Transmission electron microscopy demonstrated an increase in the number of these concretions, even after short infection periods. In the extremely swollen parts of the tubules there was a reduction in basal cell interdigitations, mitochondria, and microvilli. B. triatomae (but also gram-negative bacteria) occurred only in these swollen parts.