Our understanding of snake biology is heavily biased towards species and populations occurring at higher latitudes. In particular, little information is available concerning the biology of the numerous species of Mexican rattlesnakes. We studied the reproductive ecology of female Mexican lance-headed rattlesnakes Crotalus polystictus in a montane (c. 2500 m a.s.l.) valley of the Rio Lerma, in the Mexican state of México. We collected data from 162 different females and 203 litters over 4 years (2004–2007). Parturition coincided with summer monsoon rains, with the majority of females giving birth in late June and early July. Larger females and females gestating larger litters typically gave birth earlier in the summer than did smaller females and females gestating smaller litters. Some females matured rapidly; 26 females reproduced as 3-year olds, 17 as 2-year olds and a single female reproduced at 1 year of age. Females commonly reproduced in consecutive years. Litter size and mean neonate size increased with maternal body length; however, the relative clutch mass did not vary with female size. The mean litter size was 7.3 neonates (range 3–15), and the mean neonate body length (snout–vent length) and mass were 198 mm and 8.7 g. Neonate size varied less than did other litter characteristics. Rapid maturity, frequent reproduction and synchronization of parturition with seasonal precipitation are consistent with previously observed patterns of snake reproduction at lower latitudes.