Haploid auxotrophic mutants were produced from C. tropicalis, and protoplast fusion was induced by polyethylene glycol. The resulting nutritional complementation was due to heterokaryon formation and, at a much lower frequency, to spontaneous diploidization. During cultivation, heterokaryotic clones regularly gave rise to heterozygous diploids from which, in turn, haploids could be isolated. The technique of protoplast fusion gives an opportunity for genetic analysis of this and similarly asexual fungal species.