The heavy metal cadmium is very toxic to biological systems. Although its effect on the growth of microorganisms and plants has been investigated, the response of antioxidant enzymes of Aspergillus nidulans to cadmium is not well documented. We have studied the effect of cadmium (supplied as CdCl2) on catalase (CAT), superoxide dismutase (SOD) and glutathione reductase (GR). 0.005 mm CdCl2 had a very slight stimulatory effect on the growth rate of A. nidulans, but at concentrations above 0.025 mm, growth was totally inhibited. The accumulation of Cd within the mycelium was directly correlated with the increase in the concentration of CdCl2 used in the treatments. Although a cadmium-stimulated increase in SOD activity was observed, there was no change in the relative proportions of the individual MnSOD isoenzymes. Higher concentrations of CdCl2 induced a small increase in total CAT activity, but there was a major increase in one isoenzymic form, that could be separated by gel electrophoresis. GR activity increased significantly following treatment with the highest concentration (0.05 mm) of CdCl2. The increases in SOD, CAT, and GR activities suggest that CdCl2 induces the formation of reactive oxygen species inside the mycelia of A. nidulans.