Chagas' disease, which is caused by the protozoan parasite Trypanosoma cruzi, is a public health problem in South America affecting millions of people, and more recently several thousands in countries where the disease is not endemic. Due to the magnitude of the problem, finding a cure for this disease remains a major challenge. The aim of this study is to evaluate the trypanocidal activity of ergosterol peroxide (5α, 8α-epidioxy-22E-ergosta-6, 22-dien-3β-ol) isolated from Pleurotus ostreatus (Jacq.) P. Kumm. f. sp. Florida. The ergosterol peroxide showed strong trypanocidal activity on the intracellular form of T. cruzi. Ergosterol peroxide had an inhibitory concentration (IC50) of 6.74 µg/mL on T. cruzi, but showed no lytic action on erythrocytes and no cytotoxic effect on mammalian cells at concentrations higher than 1600 µg/mL. The interaction of Trypanosoma cruzi with ergosterol peroxide in vitro resulted in a strong lytic activity possibly due to the disruption of the parasite membrane. This is the first report of trypanocidal activity, a new biological property of ergosterol peroxide isolated from Pleurotus ostreatus (Jacq.) P. Kumm. f. sp. Florida. Copyright © 2011 John Wiley & Sons, Ltd.