Peninsula ribbon snakes, Thamnophis sauritus sackenii, were collected for 14 months. Of 90 individuals collected and examined, 42 were females, 29 were males, and 19 were neonates. Females had a larger body length than did males. The smallest, sexually mature female had a snout-vent length of 428 mm. Mating occurred from March through May. Parturition occurred from June through October. Vitellogenesis began in spring and ovulation occurred between April and May. Spermatogenic activity occurred from August through December and testes were regressed during other months.