Few studies have evaluated the joint effects of elevated temperature and pCO₂ on marine organisms. In this study we investigated the interactive effects of Intergovernmental Panel on Clinate Change predicted temperature and pCO₂ for the end of the 21st century on key aspects of larval developm,ent of the American lobster, Homarus americanus, an otherwise well-studied, iconic, and commercially prominent species in the northeastern United States and Atlantic Canada. Our experiments showed that larvae (stages I-III) and postlarvae (stage IV) reared in the high temperature treatments (19º) experienced significantly lower surivial, developed twice as fast, and had significantly higher oxygen consumption rates, than those in ambient treatments (16º). Understanding how the most vulnerable life stages of the lobster life cycle respond to climate change is essential in connecting the northward geographic shifts projected by habitat quality models, and the underlying physiological and genetic mechanisms that drive their ecology.