The swimming response of Daphnia pulicaria to polarized white light was examined. This response was employed as a behavioral bioassay to investigate the effects of sodium bromide (NaBr) on Daphnids physiology. The control response to linearly polarized white light involved an orientation at 90° to the e‐vector. With the addition of NaBr the response became more random. An EC₅₀ (concentration which eliminates the response in 50% of the test individuals) of 1.31 x 10^‐1 M NaBr was calculated using the statistic von Mises K. A possible mechanism to account for the toxicity of this mild sedative is the blocking by the bromide ion of chloride channels involved in transmitting nerve impulses.