Abstract For 237Pu accumulation from seawater, the whole body concentration factors (CFs) for juvenile lobster at 18°C did not exceed 250 over the 49 days of the study. Most 237Pu was accumulated by the gills and exoskeleton. Since homarid lobsters eat their exuviae after ecdysis, the whole body 237Pu content of recently moulted lobsters was also a function of the amount of 237Pu ingested with the exuviae. Consequently, organ/tissue content and CF values for individual lobsters reflected both the time since their last moult and their exposure time to 237Pu. These factors precluded the calculation of mean whole body CF values or the accurate estimate of a mean steady state (C 55) CF value. Juvenile lobsters retained significantly more 237Pu from labelled food at 7–10°C than at 18°C. At 7°C, the biological half time for assimilated 237Pu was about 28 days, at 10°C about 15 days and at 18°C about 8 days. Very little 237Pu was transported from the gut to the internal tissues (about 1%); most was retained in the hepatopancreas.