1. Neonate growth rate variation between low- and high-altitude populations of Sceloporus jarrovi was investigated using a common garden experiment to investigate the relative importance of proximate and ultimate causation to explain interpopulation variation in growth rates. 2. There were no differences between source populations in snout-vent length (SVL) growth rate, but there was a significant interaction of source population with sex. Low-altitude males grew more slowly than low-altitude females (0.136 mm day-1 vs 0.248 mm day-1), whereas the reverse was true for high-altitude individuals (0.185 mm day-1 vs 0.164 mm day-1). Males from both populations did not have different SVL growth rates, but females did. 3. There were no significant differences in body mass (BM) change between populations, but the sex-source population interaction was significant. 4. Neonates showed litter effects in SVL, BM. SVL growth and BM change. These differences could not be explained by maternal effects due to female body mass or litter size, suggesting a genetic basis for growth rate differences. 5. The implications of these results for genetic differences in individual growth rate and for other life-history charactenstics are discussed.