A null pigment mutant was isolated from Aspergillus nidulans and characterized. The mutant developed colorless conidiophores, conidia and ascospores and never secreted any diffusible pigment into the medium. Its growth and differentiation were retarded as compared with those of wild type, which might be due to the retardation of hyphal branching. The hyphal branching, growth and differentiation were far more delayed on the medium containing mineral salts. The mutant gene, designated as npgA1, was a recessive allele and located right to galD5 by the distance of 12.5 map unit on the linkage group I. The npgA gene was epistatic to all the conidial color mutant genes and complemented by wild type allele in heterokaryon. The npgA+ gene may function in both vegetative and differentiating stage and be required in hyphal branching and pigment synthesis. The mutant produced normal levels of three phenol oxidases(laccase I, laccase II and N-acetyl-6-hydroxytryptophan oxidase), which indicates that the npgA+ gene may not regulate the genes involved in the synthesis of pigments in various differentiating organs, rather it may function in the formation of substrate for the synthesis of pigments in hyphal and spore walls.