The influence of Cd(II), Cu(II) and Zn(II) on growth and thiol production in an aquatic hyphomycete (Articulospora tetracladia) and a zygomycete (Mucor racemosus) were studied. Growth of a strain of M. racemosus, isolated from effluents contaminated by heavy metals, was not inhibited by Cd concentrations of up to 100 μM. M. racemosus was more sensitive to Cu and Zn. The production of SH-containing compounds by M. racemosus was positively correlated with Cd concentration in the nutrient medium. By contrast, glutathione concentration in the mycelium was negatively correlated with Cd concentration. Addition of Zn or Cu to the medium had no significant effect on mycelial levels of thiol compounds or glutathione. One strain of A. tetracladia, isolated from a clean stream (At-BB), significantly increased the production of thiol compounds and of glutathione in the presence of Cd; the other strain from a Cu enriched stream (At-CS) showed no significant response. Addition of Zn or Cu did not significantly change levels of thiols or glutathione, with one exception: glutathione levels in At-CS were negatively correlated with external Cu levels. Declining glutathione levels in M. racemosus exposed to Cd was coupled with the appearance of several phytochelatins. No such compounds were found in A. tetracladia.