Isolates of Pleurotus cystidiosus and P. smithii were studied to verifie the occurrence of P. cystidiosus instead of P. smithii in South America. The two species are mainly separated by the growth rate of the anamorph in culture, the morphology of the anamorph and teleomorph, intercompatibility tests, and genetic variability. In order to see if the isolate found in Brazil and previously identified as Antromycopsis macrocarpa (the anamorph of P. cystidiosus) belongs to P. cystidiosus, a species with a world wide distribution, or to P. smithii which is restricted to Mexico and South America, or if P. cystidiosus and P. smithii are the same species, isolates of different geographic origins were studied. Growth rate in culture, mono-dikaryotic matings, and genetic variability determined by RADP were investigated. The results show that the criteria used to separate the two species are unsatisfactory, and that P. smithii should be considered a synonym of P. cystidiosus; this extends the distribution of this later species to Central and South America.