A fluffy white mycelial variant of low pathogenicity developed from the normal sporulating pathogenic fungus Drechslera teres growing on P.D.A., V-8 agar, M.Y.A., and Czapek agar. Factors such as the amount and type of carbon and nitrogen compounds present influenced the production of the mycelial variant, which was less virulent than the original isolates.The two forms of this fungus differed in the amount of growth according to temperatures, pH values of the media, and nutritional conditions. The mycelial form arose more frequently in conidial cultures grown in media of a high sugar content. Growth of the conidial form was not so good as that of the mycelial form on media containing amino acids.There was some loss of pathogenicity of the conidial culture after it had been grown and transferred eight consecutive times on media containing methionine, cysteine, or asparagine. The pathogenicity of the mycelial form was unaltered regardless of its nutrition.Factors which lead to the development of the mycelial variant are discussed.