Abstract The selenate (VI) and selenite (IV) forms of selenium were found to be acutely toxic to the freshwater green alga Selenastrum capricornutum Printz. The concentrations causing 50% growth inhibition (EC50) in 24 hr were 61.5 mg Se/l for selenate and 143 mg Se/l for selenite. Subsequent EC50 values for selenite were; 100 mg Se/l at 2 days; 96 mg Se/l at 4 days; and 65 mg Se/l at 6 days. Selenate was more toxic, producing 88% growth inhibition in 6 days at 40 mg Se/l compared to 18% for selenite at the same concentration. The relationship between growth inhibition and selenium concentration was found to be linear over the range of concentrations tested for both the selenate and selenite forms.