Abstract Dissolved inorganic arsenic and phosphate levels present in an enclosed experimental ecosystem have been studied during the course of a natural spring diatom bloom. Whilst phosphate levels dropped rapidly to almost zero as the bloom developed, arsenic levels remained constant even when arsenate levels exceeded the dissolved phosphate. Certain diatom species ( Thalassiosira spp., Chaetoceras spp. and Skeletonema costatum) therefore appear to be capable of discriminating, either by uptake or excretion, between phosphate and arsenate. The implications of such findings are discussed.