We studied the ecology of the checkered garter snake, Thamnophis marcianus, in a desert grassland in southeastern Arizona. The adult sex ratio was 0.67 M:F and varied significantly among seasons. As with other species of Thamnophis, adult females were significantly larger than males in SVL and body mass, and females also had longer jaws. Activity occurred both during the day and nighttime, but was confined to aquatic areas or their immediate vicinity. Mating occurred in late March and females gave birth to a single brood of an average of 15 offspring in late May and early June. The timing of birth in this population was among the earliest on record for any live-bearing snake in North America and was much earlier than for other checkered garter snake populations. In comparison with populations of T. marcianus from northern and southern Texas, females from Arizona had larger maternal body sizes and their offspring were larger as well; conversely, we found no significant differences in brood size among localities once maternal body size was taken into account. The implications of early timing of birth and geographic variation in reproductive traits are discussed.