Nutritional deficiencies are a very common phenomenon, and consumers generally face food that is not optimally suited for their needs. Especially herbivores are habitually confronted with food of inferior quality, usually a result of too�low nutrient concentrations in plant material. Waterfleas of the genus Daphnia are good model organisms to study the effect of inferior quality food, and how animals deal with this. We tested the effect of algae to which we had given different phosphorus contents on both life history and feeding parameters of Daphnia magna. Phosphorus content of the algae strongly affected both the growth rate and the feeding activity of the daphniids. Feeding activity increased with declining food quality (increase in C:P ratio of the algae), whereas growth rates were maximal at intermediate C:P levels. We conclude that the direct limitation of phosphorus is a very important factor determining food quality for zooplankters. Daphniids counterbalanced lower P content of their food by spending more C (energy) on acquiring this limiting resource. This implies that when Daphnia are given phosphorus�limited food both the addition of phosphorus and the addition of carbon (energy) should increase the growth rate of the animals (co�limitation). The influence of the phosphorus content of the food on the feeding activity of Daphnia offers a mechanistic explanation for the observed homeostasis in daphniids.