Colletotrichum lagenarium and Alternaria alternata produce a dark pigment, melanin. The C. lagenarium PKS1 and A. alternata ALM genes are involved in polyketide synthesis in the melanin biosynthesis pathway. PKS1 encodes a type I polyketide synthase. For functional comparison of the ALM gene with the PKS1 gene, we examined whether the A. alternata ALM gene could restore melanin synthesis in C. lagenarium albino mutant (Pks1(-)). The ALM gene transformed the albino mutant (Pks1(-)) to melanin-producing phenotypes, designated CAL transformants. The pigment intensity of both melanized colonies and appressoria of CAL transformants was weaker than that of the wild type. Ultrastructural studies of the cell walls of appressoria demonstrated that CAL transformants formed an outer melanized layer, as did the wild type. On the other hand, the thin inner and middle layers were less electron-dense than those of the wild type. CAL transformants were able to penetrate cellulose membranes as effectively as the wild type. By contrast, the penetration frequency of CAL transformants on cucumber cotyledons was remarkably reduced compared to that of the wild type. During conidial germination, the PKS1 transcript accumulated de novo in both the wild-type and CAL transformants after the start of conidial incubation. On the other hand, ALM transcript accumulated in conidia of CAL transformants before the start of conidial incubation.