The relationships between concentrations of the heavy metals Pb, Cd, Cu, Zn, Co, Ni, and Ag in nine bivalve molluscs (Arca ventricosa, Chama isotoma: Lithophaga teres. Pinctada margiritifera, Pycnodonte hyotis. Spondylus ducalis. Modiolus auriculatus, Trichomya hirsuta, Ustularca renuta) and their environment were studied. The variation in heavy metal accumulation in these field samples and the effects on metal levels and condition of the organisms after relocating for 2 months four of these species (T. hirsuta, M. auriculatus, S. ducalis and C. isotoma), were also studied. The relocated animals maintained or improved condition as judged by the condition index (i.e. ratio of tissue wet weight to shell diameter or weight) during the expefimental period. Levels of Pb and Cu in relocated T. hirsuta varied directly with environmental levels but elevated levels of Zn in the environment were not indicated by this organism. Accumulation of Pb and Cd by T. hirsuta was linear with time and increased in direct proportion to the external concentration of these metals. No loss of either metal was detected after 12 days in 'normal' seawater. The results indicate that T. hirsuta would be a suitable organism for detecting increased levels of heavy metals, except Zn, in the environment.