The effect of heavy metals on exoenzymatic activities (a-glucosidase and exoproteolytic) was studied in sediments off the coast of Catalonia (NE of Spain). Sample areas were chosen spanning a wide range in sediment conditions and waste inputs. Heavy metals were associated with organic matter present in the sediment. Covariance analyses showed that the expected relationship between organic matter content and the enzymatic activities of coastal benthic bacterial communities was altered due to the effect of heavy metals. Laboratory experiments showed the inhibitory effect of heavy metals on exoenzymatic bacterial activities. Thus, although more substrate was potentially available to heterotrophs, enzymatic degradation of organic matter was negatively affected because of the inhibitory effect of the heavy metals. A direct result of this perturbation is that the turnover of polymeric compounds is much slower in polluted areas.