Copper and zinc salts act synergistically and have inhibitory effects on the growth of the diatomsAmphora coffeaeformisandAmphiprora hyalina. The concentrations required to cause a significant reduction in growth yields indicate that the isolates of the two species used are copper and zinc tolerant. Some growth stimulation was found in lower concentrations of the two metals. Cellular levels generally increased with increasing concentrations in the external medium, resulting in increased growth inhibition. Growth yields and cellular copper concentrations were found to be related to calculated cupric ion activities in the medium. Relative inhibitory and cellular concentrations of copper and zinctor A. coffeaeformisandA. hyalinasuggest that the latter species may be more sensitive to CuC12‐2HjO and more tolerant to ZnCl2than the former. The results suggest that inclusion of ZnO as an additional biocide in self‐polishing copolymer antifouling paints would result in more effective antifouling action.