We studied demography of a population of viviparous lizard, Sceloporus bicanthalis, from Nevado de Toluca Volcano, Mexico, using mark-recapture methods. Age structure of this population varied seasonally due to an increase in number of juveniles over summer, although neonates were observed in every month. Estimated average density was 304 individuals/ha. Snout-vent length at sexual maturity in viviparous S. bicanthalis was 31 mm in males and 32 mm in females. Males survived longer than females, with an expected lifespan of 15 months, compared to 8 months for females. Net reproductive rate of this population was 1, suggesting it was at equilibrium, while generation time was 7 months. This high-elevation population has a semelparous life cycle.