The inhibition of growth of a wild strain ofNeurospora crassa by Cu2+ is counteracted by histidine, histidine methyl ester, histidinol and Mn2+. In the presence of Cu2+, the total free amino acid content decreased by 30%. The decreased free amino acid pools of arginine, histidine and tyrosine were restored on the addition of Mn2+. Histidinol phosphate phosphatase showed a decrease in activity in the presence of Cu2+. This inhibition was reversed on the addition of excess Mn2+. The data suggest that copper toxicity in the mould is due to suppression of histidine biosynthesis.