Twenty-three colour mutants of Cochliobolus heterostrophus were obtained by mutagenesis. Mutants at six loci were identified; alb1 (type-1 white), alb3 (type-2 white), sal1 (salmon), brn1 (brown), pgr1 (pale green), and scr1 (scarlet). Crossing experiments indicated that sal1 and pgr1 were in the same linkage group 5·8% apart, and that alb1, alb3 , and brn1 were closely linked. No linkage was observed between the three closely linked genetic markers ( alb1, alb3 , and brn1 ) and sal1 or pgr1. scr1 was independent of both of these linkage groups. Furthermore, no linkage was found between these loci and the mating type ( MAT1 ) locus. Accumulation of scytalone in the sal1 mutant indicated that the melanin of this fungus is formed from 1,8-dihydroxynaphthalene, derived in turn from pentaketide, and that the genetically deficient step in this mutant was at the conversion of scytalone to 1,3,8-trihydroxynaphthalene in the melanin biosynthesis pathway. The pgr1 mutant failed to form melanin from 1,8-dihydroxynaphthalene, which suggested that the deficiency of the pgr1 mutant is involved in oxidation of 1,8-dihydroxynaphthalene. The conversion of 1,3,8-trihydroxynaphthalene to vermelone may be blocked in the brn1 mutant. The defect of the alb1 mutant seemed to be involved in some process prior to scytalone formation. The alb3 mutant, which was not coloured by scytalone, was one of the melanin-deficient mutants, but its genetically blocked point was not characterized.