Protoplast fusion and nutritional complementation between auxotrophic mutants ofMicrosporum gypseumhas been achieved. The highest recombinant frequency (1%) was obtained between fused protoplasts of phenylalanine and glutamic acid auxotrophs using a solution that contained 35% polyethylene glycol, 1 M KCl, 0.05 M CaCl2and 0.05 M glycine with pH 6.1. Among different minimal media tested, minimal medium 2, which had a high sulphur content, supported the best growth of the fusion products. The hybrid colonies were slow growing, irregular in shape, with diminished sporulation and a wide range of pigmentation. Most of the hybrid colonies on complete medium segregated readily into two types of parental auxotrophs by producing distinctive sectors. These segregations suggested that the colonies were heterokaryons. This study was undertaken to elucidate the mechanisms of protoplast fusion and to understand complementation of nutritionally deficient mutants of dermatophytes.