Polybrominated diphenyl ethers form a group of highly hydrophobic and persistent chemicals that are detected in various environmental matrices, such as soil, water, sediments, and biota. Although a variety of biological effects have been documented in vertebrates, the effects in invertebrates seem to be largely unknown. The objective of the present study was to determine the toxic effects of 2,4,2',4'-tetrabromodiphenyl ether (BDE 47) on the growth of the marine diatom Skeletonema costatum and on the parthogenetic reproduction and filtering activity of the freshwater crustacean Daphnia magna. The results showed that BDE 47 caused growth inhibition in S. costatum (no-observed-effect concentration [NOEC], 6.6 microg/L; median effect concentration, 70 microg/L; 95% confidence interval, 64-77 microg/L) and depressed the reproductive output of D. magna (NOEC, 14 microg/L). No effects were seen on the filtering rate of D. magna at any of the concentrations tested. Although sublethal toxicity was observed at low-microg/L levels, documented environmental water concentrations are many orders of magnitude lower, thus suggesting that BDE 47 is of minor risk to these organisms through direct water exposure.