Invertebrate herbivores are often faced with food of different quality, affecting growth, and reproductive rates. Temperature also has a strong influence on these rates, but how temperature and food quality interact is largely unknown. We investigated the interaction between temperature and food quality on the developmental rates of a keystone plankton herbivore, the calanoid copepod Acartia tonsa. We fed the copepod along a gradient of five food qualities from phosphorus-limited to phosphorus-replete algae, exposing them to eight different temperatures from 8°C to 22°C in a full factorial design. We observed that temperature and food quality as well as their interaction significantly affected copepod growth. Food quality effects were strongest at low temperatures and decreased with increasing temperatures. Our results suggest that ectotherms need more carbon relative to phosphorus at higher temperatures to meet their metabolic demands. Thus, the threshold elemental ratio for growth should be higher at higher temperatures and a higher probability of carbon limitation for secondary production when it is warmer.