Joint toxic effects of copper and zinc were studied in the terrestrial worm Enchytraeus crypticus (Westheide and Graefe) (Oligochaeta, Annelida). Animals were exposed in OECD artificial soil. Sublethal toxicity was judged by effects on reproduction. Metals were applied singly or in binary mixtures. Observed effects were compared with effects expected from simple similar action (concentration addition), by recalculation of metal concentrations in toxic units. Exposure of the worms was quantified with body concentrations and with external concentrations (total, extractable, soluble). The observed joint effect was similar to concentration additive when judged by external concentrations and less than concentration additive for body concentrations. This difference is attributable to interactions among metals during sorption to soil and during uptake. Copper reduced the sorption of zinc to soil, but copper sorption was inert for zinc addition. Zinc uptake from the soil solution was stimulated by copper, but copper uptake was not stimulated by zinc. Joint effects of toxicants to soil biota are partly determined by interactions outside the organism, as a result of dissimilarity between total and bioavailable concentrations. The design of joint toxicity studies in terrestrial systems is discussed with special reference to metal sorption in soils, experimental methodology, and laboratory practice. The joint toxic effect of copper and zinc for E. crypticus was of similar magnitude as found in studies with aquatic species exposed to metal mixtures.