The quality of Moroccan Atlantic coastal wasters was monitored from 1993 to 1997 by measuring hydrological parameters (dissolved oxygen, suspended particulate matter, phosphates, and nitrates), and using the mussel Mytilus galloprovincialis as a quantitative bioindicator of cadmium, copper, manganese, and zinc contamination. Mean concentrations of dissolved oxygen and suspended particulate matter were indicative of the effects of urban and industrial discharges of wastewater, particularly at Mohammedia, Casablanca, Mehdiya, Jorf Lasfar and Safi. Stations receiving urban wastewater showed high nitrate concentrations, especially at Mehdiya, Rabat, Mohammedia, Casablanca and El Jadida. Metal concentrations in mussels showed significant variations depending on the station and sampling period. Jorf Lasfar and Safi had the highest mean concentrations for cadmium (8 and 7 μg·g–1 dry weight respectively) and copper (74 and 25 μg·g–1 dry weight). The relation between cadmium concentrations in mussels, and phosphate concentrations in water suggests that the processing of phosphate ores at these two sites is responsible for contamination. Mussels in the Mohammedia–Casablanca sector had the highest zinc concentrations (338–379 μg·g–1 dry weight), followed by those collected at Jorf Lasfar (267 μg·g–1 dry weight), and Sidi Moussa and Safi (290–301 μg·g–1 dry weight). The distribution of manganese concentrations, which were maximal in estuaries (up to 19 μg·g–1 dry weight), was indicative of terrigenous inputs. Seasonal variations in mussel metal concentrations were characterised by winter minima, and apparently related to the physiological cycle of the animal. With the exception of cadmium-contaminated areas, the quality of mussels on the Moroccan Atlantic coast is good with respect to food safety standards.