Brominated flame retardants belong to a new class of environmental contaminants. To obtain new information regarding the effects of 2,2',4,4',5-brominated diphenyl ether (BDE-99), one of the most frequently reported congeners in freshwater biota, the inhibition of algal growth of Raphidocelis subcapitara (also known as Selenastrum capricornutum) and acute toxicity to Daphnia magna were examined. The experimental design also involved a comparison with the polychlorinated biphenyl (PCB) Aroclor 1254. The uptake of BDE-99 by R. subcapitata and the transfer to D. magna (i.e., a higher level of aquatic biota in the food chain) was also assessed to verify the bioaccumulation phenomenon. After 24 h, BDE-99 appeared to be less toxic than Aroclor 1254 to D. magna, but the two compounds showed a similar toxicity at 48 h. In contrast to Aroclor 1254, BDE-99 was nontoxic to R. subcapitata at up to 100 microM, the highest tested concentration. However, the dose-dependent decrease in survival and impairment of reproduction of D. magna fed with BDE-99-treated algal culture demonstrate uptake by R. subcapitata. Because of the high persistence and bioconcentration, polybrominated diphenyl ethers as well as PCBs might be of environmental concern for years.