A methionine-dependent (meth 1) strain of Aspergillus nidulans has been used to study the inactivating and mutagenic effects of 8-methoxypsoralen (8-MOP) plus near UV light (NUV). The presence of the meth 1 locus allows reversion to methionine independence to be detected after forward mutation at any of several (at least 5) independent suppressor gene loci, and revertants can be divided by phenotype into 3 distinct classes corresponding to mutation at particular groups of suppressor loci. Treatments of conidia with either 8-MOP, NUV, visible light (VL), 8-MOP plus VL, or NUV followed by 8-MOP, show no inactivating or mutagenic effect, whereas a treatment with NUV in the presence of 8-MOP produces inactivation and mutation under similar conditions. The prior treatment of conidia with 8-MOP before exposure to NUV produces a more rapid inactivating effect, and a greater mutagenic effect, than a treatment with 8-MOP followed by immediate exposure to NUV, presumably because the prior treatment allows the 8-MOP to reach a higher initial concentration in the conidia before exposure to NUV. The results suggest that the optimum effect of 8-MOP is attained by about 5 min treatment with 8-MOP before exposure to NUV. The similar frequencies of mutation for the 3 meth 1 suppressor classes over varying times of exposure to 8-MOP and NUV indicate that the various suppressor loci are equally mutable by the treatment, in contrast to that found for spontaneous mutation, and for previously reported treatments with chemical mutagens. The frequency of meth 1 suppressor mutations induced by 8-MOP prior to NUV is about 12 times greater than a corresponding treatment with diethyl sulphate at approximately the same level of conidial survival.