Duckweed (Lemna minor), a small vascular plant, grows rapidly, is sensitive to a wide variety of toxicants and is easy to culture. A method is described that measures duckweed frond growth and chlorophyll levels as indicators of growth inhibition. The method requires a small testing volume (15 ml) of reconstituted test medium that is prepared by using commercial grade soil to promote frond and chlorophyll production. A variety of single toxicants, industrial effluents and wastewater treatment plant effluents were tested. For most chemicals, the duckweed was as sensitive as other frequently used test species. The comparative toxicities to Ceriodaphnia dubia and fathead minnows (Pimephales promelas) with various effluents indicates that, for some industrial effluents, duckweed was the most sensitive indicator used.