Newly hatched zebrafish (Brachydanio rerio) were exposed to eight compounds of selenium to establish their relative toxicities. The compounds examined included the organoselenides of methionine and cystine, the inorganic selenite salts of sodium and potassium, selenium dioxide, and the inorganic selenate salts of calcium, sodium, and potassium. The selenite compounds were more toxic to zebrafish than the selenates; the 96-h concentrations lethal to 50% of the fish ranged from 15 to 23 μg/ml for the selenites and from 81 to 82 μg/ml for potassium and sodium selenate. The limited solubility of selenocystine and the significant losses of selenomethionine from the test solutions did not allow comparable estimates to be made for the selenides. These differences in toxicity between the selenites and selenates were also demonstrated when selenium levels required to produce a 5% mortality at the end of the 10-day observation period were estimated. There was a marked difference in toxicity between the organoselenides at this level, with the toxicity of selenocystine being similar to the selenates, and selenomethionine being the most toxic of all the compounds tested.The probable role of selenium in the aquatic environment is also examined. The increasing awareness of this potentially significant pollutant, and its antagonistic nature with seven toxic or potentially toxic elements is also noted.