In the presence of light and oxygen, rose bengal causes oxidative damage to Trypanosoma cruzi. The production of lipid hydroperoxides was demonstrated by thin-layer chromatography, and severe ultrastructural alterations compatible with an increased permeability of the cells, which led to gradual osmotic swelling and ultimately to lysis, were observed by electron microscopy. As a result of this treatment, the infectivity of T. cruzi trypomastigotes in mice was abolished. In addition, under anaerobic conditions, rose bengal was found to undergo a one-electron reduction in intact T. cruzi epimastigotes to produce a carbon-centered free radical as demonstrated by electron spin resonance spectroscopy. The formation of this radical was also enhanced by light.