Miconazole and econazole, two fungicide imidazole derivatives, completely inhibited growth of Trypanosoma cruzi (Tulahuen strain) at concentrations of about 20 μM. Culturing of T. cruzi in the presence of lower doses of imidazole derivatives produced: decrease of 5,7-diene sterol content in epimastigotes (including ergosterol); disappearance of the nuclear chromatin, vacuolization and decrease in the electron density of the cytoplasm; selective surface alterations as revealed by an increased response to wheat-germ- and phytohemagglutinin. At variance with the effect of miconazole on Candida (De Nollin et al. (1977) Antimicrobiol. Agents Chemother. 11, 500–513), miconazole and econazole, under the experimental conditions used, did not increase the rate of hydrogen peroxide generation by T. cruzi.