We studied how chemicals obtained as filtrates from algal monocultures (algal chemicals) and from rotifer cultures with or without algae (rotifer chemicals) affected feeding rates of the rotifer Brachionus calyciflorus on its food algae, both directly and indirectly (through chemical-induced changes in algal morphology). Algal chemicals had a strong stimulating effect on the feeding rate of B. calyciflorus, but these effects were counteracted by rotifer chemicals. In functional response experiments, rotifer chemicals lowered maximum ingestion rates and had strong effects on assimilation rates and assimilation efficiencies of B. calyciflorus, probably due to the release of unspecific (auto)toxic metabolites. Furthermore, rotifer chemicals induced colony formation in the food alga Scenedesmus obliquus. Above the optimum particle size for ingestion by B. calyciflorus, larger algal colony sizes increased the food-handling time, thus lowering ingestion and assimilation rates. Through their effects on trophic interactions, infochemicals may play a role in structuring and the functioning of aquatic food webs.