Hatchling Cuban iguanas (Cyclura nubila) were reared for 38 weeks under three experimental conditions: nighttime temperatures of 15, 25, and 35 °C. All groups experienced a temperature of 35 °C during the day, and all were given food and water ad libitum. Growth rate (in terms of mass and length) increased with increasing nighttime temperature. The iguanas held at constant 35 °C were also more robust (defined as mass/length); this difference was not explained by the amount of fat. Passage time of food through the gut was shorter for the constant temperature group, which also consumed more food per day. There was no difference in digestive efficiency among groups, and the differences in growth rates were a result of the differences in food consumption. Constant temperature was apparently not detrimental for this tropical, herbivorous lizard. More data are required before useful generalizations concerning voluntary hypothermia in reptiles can be made.