The growth of individuals from a copper-tolerant and a non-tolerant population of Ectocarpus siliculosus (Dillw.) Lyngbye, in media containing copper (0.10 mg 1-1), cobalt (0.10 mg 1-1), zinc (0.25 mg 1-1) or cadmium (1.50 mg 1-1) was measured using a wet wt method. The copper-tolerant population was found to have an increased tolerance to cobalt and zinc, in addition to a high tolerance to copper. It is believed that the copper tolerance mechanisms, or other physiological processes, typical of the fouling population, are the basis of this co-tolerance. The importance of a general tolerance to heavy metals in a major fouling alga is noted.