The only major industrial outfall in the Usk Estuary is from the British Steel Company Orb Works which discharges effulent including a variety of metals, oils and neutralised pickling liquor. Previous studies have indicated that bioaccumulation of some metals by the alga Fucus vesiculosus may be attributable to this effulent and so a study of the benthic fauna was undertaken at 14 sites within a distance of 1.3 Km during July 1984 to assess the biological effects of the effulent. In addition bioaccumulation of metals by the polycheate Nereis diversicolor was also investigated. Seventeen macroinvertebrate taxa were collected (range 3-11 per site), the most widespread species being Nereis diversicolor and Macoma balthica, although the former species occured at relatively low densities within 200 metres of the discharge possibly indicating a deleterious effect as a result of the effulent. Copper concentrations in tissues of Nereis diversicolor were higher than in the sediments and were related with distance from the outfall, although tissue and sediment concentrations were generally similar to other estuaries such as the Humber. It was concluded that the effulent does have a detrimental effect on the fauna locally, although specific toxic substances were not identified. It is reccommended that proposed benthic studies in estuaries are supplemented by more widespread bioaccumulation studies, screening of effulent toxicity (for example Microtox system) and measurement of simple growth parameters of transplanted fauna.