We report the first information on seasonal abundance, sex ratio, body size, reproduction, and diet for a population of Thamnophis scalaris from the State of Mexico. Data were collected during 1992 and from 1998 to 2000 at Toluca. Thamnophis scalaris showed a bimodal peak of activity in spring-early summer and in autumn. The sex ratio of adults was significantly different from 1:1, but can be misleading because of the unknown natural history of T. scalaris. Sexual size dimorphism was only apparent in relative tail length of adults. Four litters were born from July 1 to 17. Mean litter size was 7.0 ± 1.4 snakes per litter. Male and female neonates were of similar snout-vent length (SVL) and mass at birth. The sex ratio of neonates was 1:1. Of the stomachs examined, 81% contained earthworms and 19% contained vertebrates. Vertebrates (lizards and viperine snakes) were ingested only by T. scalaris >40.0 cm SVL. This suggests an ontogenetic shift in the diet of T. scalaris. The mean vertebrate prey mass was 21.4 ± 10.0% of snake mass. No differences were found in prey type between the sexes.