Protoplasts of a Schizophyllum commune tryptophan auxotroph (trp1), deficient in indole-3-glycerol phosphate synthetase (IGPS), were transformed to trp+ with plasmid DNA containing the Schizophyllum TRP1 sequence. Efficiencies up to 30 transformants per microgram of plasmid DNA were obtained. Southern blots reveal that the transforming DNA is integrated in chromosomal DNA. The trp+ phenotype of transformants is stable in meiosis and mitosis. Transformants possess IGPS activity comparable to wild-type cells.