The uptake of nickel by U. muhlenbergii (Ach.)Tuck. has been investigated. The data suggest that the uptake is physicochemical in nature. Metabolic inhibitors caused no significant decrease in nickel uptake. Dead thalli accumulated the metal to a slightly greater extent than live thalli. Nickel uptake was similar under aerobic and anaerobic conditions. Uptake was pH dependent and increased with temperature. The development of a model for nickel uptake by U. muhlenbergii based on thermodynamically rigorous expressions is described in this paper.