A critical literature review has been undertaken of the effects that bromate (BrO3-) has on aquatic organisms. Chronic (multigeneration) studies using four marine phytoplankton species indicate a stimulation of cell division at concentrations up to 13.6 mg BrO3- liter-1, such that the EC50 is > 13.6 mg BrO3- liter-1 (exposure period unspecified). No published data were available for any freshwater algal or phytoplankton species. For freshwater invertebrates, the available data are limited to acute studies using two species: the water flea, Daphnia magna, with a 48-h LC50 of 179 mg BrO3- liter-1; and the planarian ("flatworm"), Polycelis nigra, with a 48-h LC50 of 2258 mg BrO3- liter-1. For saltwater invertebrates, there is a greater variety in the spread of species sensitivity to bromate. For crustaceans, the lowest reported acute value is a 24-h LC50 of 176 mg BrO3- liter-1 for mysid shrimp. For molluscs, the published data are somewhat contradictory, with different workers reporting EC50 values to oyster embryo-larvae of either 0.05-0.1 or 30 mg BrO3- liter-1. Following repeated attempts to resolve these differences using the Pacific oyster (Crassostrea gigas) embryo development assay, a 24-h EC50 of 170 mg BrO3- liter-1 was observed. Reported 96-h LC50 values for juvenile saltwater fish range from 427 to 512 mg BrO3- liter-1, while a 10-day LC50 of 279 mg BrO3- liter-1 is also reported. For fish early lifestages, newly hatched larvae are more sensitive than embryo hatching, with a larval 96-h LC50 of 31 mg BrO3- liter-1 being observed. Less severe effects have been reported for older larvae (pro-larvae), for which a 96-h LC50 of 404 mg BrO3- liter-1 is reported. In conclusion, a number of studies provide evidence for the acute toxicity of bromate to invertebrates and fish in the lowest region of 30 mg BrO3- liter-1. Applying the factor of 10 used to extrapolate from acute to potential chronic toxicity, the available data suggest that to protect aquatic organisms from long-term adverse effects, concentrations should not exceed approximately 3.0 mg BrO3- liter-1.