Thermoregulatory capacities of young Ring-billed Gulls (Larus delawarensis) were studied within the first 24 hr after hatching. These birds controlled body temperatures (Tb) effectively over 2-hr tests conducted at single ambient temperatures (Ta) baetween 20 and 45.6 deg. C. Increasing heat production constitutes an important element in this control between 20 and approximately 34 deg. C. Metabolic rates of hatchlings at the lower temperature were approximately double those at the higher, where minimal rates for euthermic birds occurred. Electromyographic observations indicated that shivering contributes importantly to increasing heat production. The chicks also possess some capacities for controlling insulation. They additionally displayed a well-devloped panting response at high Ta’s, which contributed to effective evaporative cooling. Below 20 deg. C., Tb varied directly with Ta at the end of 2-hr tests, but the birds remained at least 5 deg. C warmer than their surroundings even when exposed to 10 deg. C. The chicks tolerated hypothermia. Field observations indicated that hatchling Ring-billed Gulls in the absence of their parents can be exposed to either heat or cold stress on June days, depending on whether they are in the sun or shade. Comparisons of the thermoregulatory capacities of hatchling Ring-billed Gulls with those of hatchlings of other species are presented. Among gulls, some interspecific variation seems to exist in the thermogenic capacities of the chicks.