Tadpoles of the American toad, Bufo americanus, undergo a diel cycle of distribution and activity. Tadpoles spend the night scattered throughout all but the shallowest parts of ponds, where it is cooler than in nearby deeper areas. They begin swimming and feeding more frequently as light intensity increases in the morning, and when shallow areas begin to warm, they move into them. Most are in aggregations in the upper 1-2 deg. of temperature gradients through midday. During late afternoon, they move from shallow water and by evening are scattered and inactive. This diel cycle is closely correlated with changes in light. Light triggers activity in the morning, and swimming and feeding are depressed on cloudy days. Diminishing light causes individuals to disperse from aggregations and becomes inactive. Once active, tadpoles are attracted to particular microhabitats by heat and light. Temperature gradients are primarily responsible for bringing tadpoles together in sufficient density for aggregations to develop, but thermal effects do not act alone in controlling aggregations.