The possibility of P limitation for zooplankton growth has many implications for understanding changes in production efficiency and feedback dynamics between consumers and resources. However, there have been no direct tests to determine whether the putative P limitation is real. To answer this question, we directly supplied inorganic P to Daphnia magna apart from food algae, Scenedesmus acutus, and then examined changes in body mass. During the period from birth to age 6 d, D. magna were fed on live algae for 19 h and placed in water of high inorganic P (4 mM: P treatment) for 5 h each day. We used P-free water as a control treatment. Growth rate estimated from initial and final body mass during the 6-d incubation was significantly larger in the P treatment than in the control treatment when Daphnia fed on P-deficient algae, whereas a significant difference was not detected between the treatments for Daphnia fed on P-sufficient algae (C : P atomic < 300). The results clearly demonstrate that Daphnia growth is in fact limited by P itself when they feed on P-deficient algae.