The marked copper tolerance of Penicillium ochro-chloron has been confirmed as has its ability to grow in solutions of high salinity. The major low molecular weight organic solutes present in P. ochro-chloron were glycerol, erythritol, arabitol, mannitol, sorbitol and trehalose. Of these, glycerol increased significantly during growth in high concentrations of Na+ or Cu2+, a 15-fold increase, relative to the control, occurring in concentrations of 0.5 M. Cell K+ increased with elevated external Na+ but decreased slightly with elevated external Cu2+. With high external Cu2+ mycelium maintained a constant level of Cu2+, with low levels of Na+ in all treatments. It is concluded that the high concentrations of glycerol induced by high external Na+ or Cu2+ had a significant osmotic effect, allowing growth in media of high osmolality. The exclusion of ions may have a gratuitous role in the copper tolerance of P. ochro-chloron.