Anthropogenic climate change is thought to present a significant threat to biodiversity, in particular to tropical ectotherms, and the effects of long-term developmental heat stress on this group have received relatively little research attention. Here we study the effects of experimentally raising developmental temperatures in a tropical butterfly. We measured survival, development time, adult body mass, and wing size of a neotropical butterfly, Heliconius erato demophoon , across three temperature treatments. Egg survival was lower in the hotter treatments, with 83%, 73%, and 49% of eggs eclosing in the 20-30°C, 23-33°C, and 26-36°C treatments, respectively. Larval survival was five times lower in the 26-36°C treatment (4%) compared to the 20-30°C treatment (22%), and we did not detect differences in pupal survival across treatments due to high mortality in earlier stages. Adults in the 20-30°C treatment had a lower body mass and larvae had a lower growth rate compared to the intermediate 23-33°C treatment, but were heavier than the few surviving adults in the 26-36°C treatment. Females were heavier and grew faster as larvae than males in the 23-33°C treatment, but there was no associated increase in wing size. In summary, high developmental temperatures are particularly lethal for eggs and less so for larvae, and also affect adult morphology. This highlights the importance of understanding the effects of temperature variation across ontogeny in tropical ectotherms.