Acute toxicity tests were carried out with three metals on the marine copepods Scutellidium sp., Paracalanus parvus and Acartia simplex. The resulting 24-h LC50 values showed that copper was the most toxic metal, with cadmium more toxic than zinc for two of the three species. Scutellidium sp. was more sensitive than P. parvus and A. simplex to all metals, LC50 values being 0.18, 0.66 and 1.09 mg/l for copper, cadmium and zinc respectively. No consistent trend in relative sensitivity was apparent for the other two species. The results were in general agreement with those for other marine copepods given by other workers. Some experimental problems in conducting toxicity tests with copepods are indicated. Application of the present LC50 data to existing water quality criteria produced unrealistic 'safe' concentrations in the case of copper. Such a finding illustrates the arbitrary nature of application factors and questions the usefulness of acute toxicity tests as the major tool for environmental protection.