Resorcinol (1,3-dihydroxybenzene) was identified as one of the major constituents of the exudate ofNuphar lutea seedlings, which were raised from seeds and cultivated under axenic conditions. The compound is released from the plants in considerable amounts (up to 15 nmol/seedling/day). Highest rates of resorcinol exudation were measured when the seedlings were incubated under physiological conditions (macronutrient and light supply) that resembled those of natural stands of the plant. An inverse correlation exists between nitrate and/or light supply and resorcinol production. Because of its generally toxic properties, resorcinol is suggested to play a role as an allelochemical in interactions between macrophytes and other organisms of the aquatic ecosystem. A first approach of resorcinol application to zooplankton and phytoplankton organisms resulted in deleterious effects against aDaphnia species. Two Cryptophyceae species reduced resorcinol concentration to zero, showing a concomitant increase of the size of starch granule enclosures. Cyanophyceae and Chlorophyceae seemed not to be affected.