1. The effects of temperature and food concentration on the fitness of Daphnia magna were tested in a 4×4 factorial flow‐through design. Food ranged from 0.1 to 1.0 mg C L−1 and temperature ranged from 15 to 30 °C. 
2. The juvenile growth rate (gj) was used to construct reaction norms for temperature at varying food concentrations. Two clones isolated from the same pond at different seasons did not differ with respect to their temperature responses. Reaction norms had the shape of an optimum curve with highest values around 20 °C. There was a significant temperature–food interaction as the temperature response was most pronounced when the food was not limiting. 
3. Differences in fitness were a consequence of different responses of physiological parameters to food and temperature. Age and size at first reproduction, as well as egg numbers, decreased with increasing temperature and decreasing food concentration. 
4. As the temperature effect was strongest at the highest food concentrations, it can be concluded that environmental warming may affect D. magna more through a temperature rise earlier in spring rather than in summer.