The capacity of Ulva rigida as a bioindicator of Zn, Cu, Pb and Cd contamination in oligohaline coastal waters was investigated. Metal concentrations in U. rigida , water and sediments and several environmental variables were measured seasonally at four stations in the coastal section of the Evros River Delta, Aegean Sea. All four metals in sediments had a significant local variation; in particular, they decreased with increasing distance from the fluvial end-member of Evros River. Mean Zn, Cu, Pb and Cd concentrations in U. rigida were 61.0, 22.4, 6.1 and 1.1 μg g -1 dry weight, respectively. This macroalga, compared to other co-occurring macroalgal species, had a high metal accumulation capacity, mainly of Cu and Cd. Local patterns of Zn, Cu and Pb in U. rigida were similar to those in sediments, while that of Cd was not; moreover, Cu and Pb in U. rigida correlated positively with their sediment contents, suggesting that this seaweed satisfies the basic prerequisite for a bioindicator of these elements in oligohaline environments. No metal in U. rigida displayed a significant seasonal variation, and there was no significant variation in ambient concentrations. A negative correlation between Zn in U. rigida and water salinity was found, probably due to both a higher metal burden in the freshwater flowing in the study area and to a decrease in Zn uptake with increasing salinity; Cd in the macroalga correlated negatively with water column ammonium.