The thermal characteristics of nest sites of the oviparous snake, Opheodrys vernalis, were studied in 1975 and 1976 near Cheboygan, Michigan. These characteristics were contrasted to the thermal regime of potential nest sites at shallow (1 cm) and deep (30 cm) locations in two constrasting habitats, open fields versus forest. On the basis of the great difference in temperatures of the two sites, it is postulated that oviparous reptiles at high latitudes or altitudes would tend to be restricted to open habitats where maintenance of higher incubation temperatures is more likely.