Samples of Mytilus edulis, together with samples of sediment and Fucus vesiculosus, were taken monthly from each of five shores on the south coast of the Shannon Estuary from March 1992 to December 1993 inclusive. Samples were digested individually and the levels of iron, zinc, manganese, cobalt, chromium and copper were determined using atomic absorption spectrophotometry. Levels of metals were lower than previously published values from Irish east coast estuaries, including Dublin, except for a few high levels in M. edulis, which did not persist in the tissues and were probably lost through excretion, spawning or detoxification. Of the metals analysed iron showed the highest levels. This was attributed in part to the background levels of iron in sediment. On some occasions high metal levels ill sediment did not lead to higher levels either in F. vesiculosus or in M. edulis, suggesting that metals in M. edulis may come from other sources, including anthropogenic inputs, in the estuary. On an international scale the levels in M. edulis of the six metals studied were relatively low in the Shannon Estuary. The longer-term monthly sampling regime followed in this Study is important since it shows that occasional high levels, which may be detected by once-off studies, are not always maintained.