This paper presents the first temperature-growth performance curves for a coral reef fish. Thermal tolerance and growth for the juvenile spiny damselfish Acanthochromis polyacanthus were measured at a range of temperatures from 15°C to 38°C. A. polyacanthus juveniles showed a critical thermal minimum at 15.5°C±0.1 and a critical thermal maximum at 38°C±0.12. Maximal growth (based on changes in length) occurred at 28-31°C, whereas weight gain was maximised at 28°C, which corresponds closely with the annual mean temperature currently experienced by these fishes in their natural environment. At temperatures >31°C the growth rate decreased markedly in length and weight up to 34°C, where fishes had negligible growth and died within 8-15 days. Sustained increases in ambient temperature (due to climate change) are expected to have significant adverse effects on these fishes. However, any effects of increasing temperature may also be offset by changing the timing of reproduction; by breeding in early spring or late summer, these fishes may still be able to exploit narrow windows of thermal optima, whereas fishes breeding in the height of summer will expose offspring to potentially lethal temperatures at critical stages during their development.