A simple and accurate flow-through system was developed for toxicity evaluation of biodegradable substances, wastewaters, and surface waters. The chronic toxicity of phenol to the daphnid Daphnia magna was studied during a 21 d exposure period, first using a semi-static and later a flow-through test. The flow-through system was advantageous especially when lower concentrations of phenol were examined. Its use provided a constant concentration of phenol in all tested samples. Exposure to phenol showed a similar influence on both survival and reproduction of daphnids; the IC25 value was 3.18 mg L-1.