Acute toxicity of hexavalent chromium to 3 estuarine ammals (Corophium volutator, Macoma balthica, Nereis diversicolor) has been studied at 3 temperatures (5, 10, 15 'C) and a range of salinities (5 to 40 %o, in 5 %O increments), at time intervals of up to 384 h. Toxicity increased as temperature increased and as salinity decreased. From an analysis of variance, the significant factors and their interactions were included in a response surface model for C. volutator and M balthica separately. The range of toxicity values for a single species under a variety of environmental conditions exceeded the previously reported range of toxicity values for several species maintained in constant conditions. It is emphasized that due account should therefore be made of environmental conditions when evaluating the toxicity of chromium, in the estuarine environment.