Cadmium in interstitial water, rather than that bound to particles, appears responsible for acute sediment toxicity to this species. The addition of small quantities of sewage sludge or an increase in the proportion of the fine fraction of sediment particles significantly reduced the toxicity of cadmium in sediment. Binding of cadmium by sediment particles may explain the presence of phoxocephalid amphipods at sites where sewage and metal pollution occur together. The cadmium sensitivity of R. abronius is similar to that of several marine invertebrates commonly used in seawater bioassays.