The effects of a commercially available anionic surfactant solution (FFD-6) on growth and morphology of a common green alga (Scenedesmus obliquus) and on survival and clearance rates of the water flea Daphnia magna were studied. The surfactant-solution elicited a morphological response (formation of colonies) in Scenedesmus at concentrations of 10–100 ll l-1 that were far below the No Observed Effect Concentration (NOEC) value of 1,000 ll l-1 for growth inhibition. The NOEC-value of FFD-6 for colony-induction was 3 ll l-1. Daphnia survival was strongly affected by FFD-6, yielding LC50–24h and LC50–48h of 148 and 26 ll l-1, respectively. In addition, clearance rates of Daphnia feeding on unicellular Scenedesmus were inhibited by FFD-6, yielding a 50% inhibition (EC50–1.5h) at 5.2 ll l-1 with a NOEC of 0.5 ll l-1. When Daphnia were offered FFD-6-induced food in which eightcelled colonies (43 9 29 lm) were most abundant, clearance rates (*0.14 ml ind.-1 h-1) were only 25% the rates of animals that were offered non-induced unicellular (15 9 5 lm) Scenedesmus (*0.56 ml ind.-1 h-1). As FFD-6 concentrations in the treated food used in the experiments were far below the NOEC for clearance rate inhibition, it is concluded that the feeding rate depression was caused by the altered morphology of the Scenedesmus moving them out of the feeding window of the daphnids. The surfactant evoked a response in Scenedesmus that is similar to the natural chemically induced defensive reaction against grazers and could disrupt the natural information conveyance between these plankton organisms.