Auxotrophs in three different races of Pyricularia oryzae were obtained following ultraviolet mutagenesis, and tested for complementarity on minimal medium. Prototrophic growth resulted between combinations of different auxotrophs of single strains, but not between strains. The growth did not result from crossfeeding, but required at least transitory heterokaryosis. Putative diploids, i.e., large prototrophic conidia, were readily isolated from the heterokaryons. In addition, conidia with nearly every possible combination of parental markers were found, showing that P. oryzae does indeed complete the parasexual cycle. The degree of genetic recombination found here may help to explain the extreme variability of this organism.