Heterokaryosis was demonstrated in the dermatophyte Trichophyton mentagrophytes. Ten biochemical mutants of this fungus, requiring, tryptophan, histidine, methionine, arginine and thiamine, were used in the experiments. Six mutant pairs with different biochemical markers formed the heterokaryotic mycelium (trp + his, trp + met arg, trp + thi, his + met arg). The heterokaryotic constitution was determined by ths dissociation of a heterokaryon into auxotrophic components in microconidial spreads and by the isolation of the hyphal tips from which the prototrophic colonies grew out. The heterokaryotic constitution was expressed by the complementation of nutritional requirements and morphologically.