The accumulation of chromium was investigated at the tissue and ultrastructural level in mussels (Mytilus edulis (L.)) in an estuary receiving leather tannery effluent and at a reference site. Mussels were also reciprocally transplanted between the estuaries to follow short-term patterns of accumulation and deputation, respectively. The digestive gland was found to accumulate the highest concentrations of chromium in transplanted mussels after 12 months. However, in native mussels (>8 years old), the gill contained the highest concentrations. The general trend in the chromium concentrations of native mussels was: gill > kidney > digestive gland > mantle > adductor muscle. Concentrations of chromium in the gill of mussels close to the tannery outfall were in the range 400 to 1,000 μg/g dry weight, compared to a maximum of 6 μg/g in reference mussels. Seasonality in chromium concentrations was seen at both sites with an early spring maximum evident in reference mussels that could be linked to spawning and an early summer peak in concentrations in the contaminated estuary, possibly as a result of augmented feeding or increased discharges. Relationships between tissue concentrations were also studied and a high correlation occurred between some tissues, for example, digestive gland-kidney and gill-adductor muscle. Deputation was generally slow in mussels transplanted out of the Colligan Estuary, although the digestive gland lost the most chromium. Therefore, it is suggested that two pools of chromium accumulation exist in the contaminated mussels, that is, via the gills (slow) and via the digestive gland (fast). At the cellular level, chromium was found in both particulate (lysosomal) and cytosolic fractions. In the gill, particulate chromium was associated with calcium and phosphorus, whereas chromium was detected in sulfur-dominated granules in the kidney. In the cytosol, binding to a high molecular weight component occurred in the gill, whereas binding in the digestive gland cytosol was dominated by a poorly resolved low molecular weight component. In addition, chromium is apparently detoxified by a separate pathway compared with iron or zinc, as judged by the differential cytosolic and lysosomal affinities of these metals.