The combined mutagenic action of five snbinhibitory concentrations(0,1.6,3.2,4.8 and 6.4 µg/ml) of trisoralen (4,5,8- Trimethylpsoralen, TMP) and five exposure times (0,10,20,30,and 40 min) to the long wave (UVA,365 nm) light was tested in conidia of the fungus Aspergillus amstelodami. The drug was incorporated together with the selective agent (8-azaguaninc) into the growth plates. The plates were inoculated with conidia, left for one hour and then exposed to UVA and incubated for the growth of the resistant mutants. Neither trisoralen by itself nor UVA alone appeard mutagenic under our experimental condition. Combination of the drug and UVA, however, all produced mutant frequencies significantly higher than the spontaneous one. 8-MOP used as a positive control and it induces frequency of mutation about 87.5 times that of the spontaneous one by plate incorporation method. The solvent, ethanol by itself or in combination with UVA , induced non mutant frequency greater than the spontaneous one.