It is well known that geographic variation oceurs in life history characteristics among populations of lizards. Variations in life histories found in some studies are possibly adaptations to environmental pressures. In southwestern México, the reproductive characteristics of the endemic species Sceloporus formosus have been poorly studied throughout its geographic range. We used specimens from scientific collections to describe male and female reproductive eyeles and sexual dimorphism of S. formosus. Other viviparous Sceloporus from high elevations show sexual dimorphism, with males being larger in many morphological characteristics than females. We therefore expected that S. formosus would show sexual dimorphism. However, we found no sexual dimorphism. Males reached sexual maturity at a smaller (47 mm) snout-vent length (SVL) than females (50 mm). There were no significant differences between sexes and months in liver mass. However, fat body mass in females was greater than in males. For males, there was significant relationship between log10-testis volume and log10-SVL. Testis volume was positively correlated with temperature and was smallest during October, November, and December. For females, there was no significant effect of month on gonad volume. Vitellogenesis oceurred from April to November. Vitellogenic follicles and embryonic development were significantly correlated with precipitation (Pearson correlation, r = 0.80, n = 10, P = 0.0081) and photoperiod (Pearson correlation, r = 0.72, n = 10, P < 0.0001). Litter size based on embryos was correlated with SVL (Pearson correlation, r = 0.82, n = 10, P = 0.0034). Relative litter mass was not correlated with female SVL (Pearson correlation, r = 0.43, n = 10, P = 0.2110). The fall reproductive pattern found in this species is typical of viviparous species from high elevation, where temperature is low, and precipitation and relative humidity are high. If so, it would suggest that convergence in reproductive cycle could be associated with a shift to montane habitats in these species.