Strontium, calcium, and, in some cases, barium have been determined in 15 species of algae, 45 species of invertebrates and 6 species of fish from the Irish Sea. The highest concentration factors for strontium in the algae were found in the Phaeophyceae; in the invertebrates among the Polyzoa, Crustacea and Echinodermata. Only two species, B. balanoides and O. fragilis exceed 100 for the whole organism, whilst mollusc shells were found as high as 235. Concentration factors for strontium of less than 1·0 were found in the muscle of fish. Discrimination against strontium, relative to calcium, compared with sea water, occurred in all organisms or tissues with the exception of the Phaeophyceae. Maximum discrimination was found in the shells of the Mollusca. In the radiological health field of radioactive waste management in the marine environment the data provides additional information, for temperate species, of the equilibrium value for the accumulation of radiostrontium from seawater and the food web. Where only 90Sr data is available, the degree of equilibrium may be established.