Experiments were conducted to determine (a) the feasibility of maintaining laboratory cultures of the infaunal phoxocephalid amphipod Rhepoxynius abronius (Barnard), (b) the relative sensitivities of cultured versus freshly collected adult animals, (c) the sensitivities of adult versus juvenile R. abronius to cadmium in sediment and (d) the effect of handling on cultured amphipods. R. abronius held in sediment in a flow-through seawater system for various lengths of time showed high survival, growth and sexual maturation for periods of up to 180 d. Cultured amphipods appeared normal and survived well (93%) under control toxicity test conditions, but were more sensitive to cadmium in sediment (LC50 = 4.4) than were freshly collected amphipods (LC50 = 8.7). Differences in cadmium LC50s between size classes (LC50 = 8.2 for juveniles and 11.5 for adults) were statistically significant, but not substantial. It is recommended that large juveniles and adults (3 to 5 mm) be used in sediment toxicity tests because they are available from natural populations throughout the year, and that amphipods be collected within 14 d of use in sediment toxicity tests to minimize the interaction between culture and contaminant stress. (Copyright (c) 1988 SETAC).