Aggregative behavior of tadpoles of the American toad, Bufo americanus, was investigated to find what kinds of aggregations occur, what factors regulate aggregative behavior and to learn how it unfolds ontogenetically. Studies in mid and late summer on 1963-5 and 1969-70 were carried out in northern Michigan. Spring and early summer work (1968-1970) was done 225 miles farther south near Ann Arbor. Field observations were combined with field and laboratory experiments. I conclude that aggregations are regulated by a complex of several factors acting simultaneously or in sequence. Larvae are activated by light and then attracted to warm microhabitats where food and other larvae (social influences) draw them into aggregations. Social interactions keep larvae together and coordinate their intragroup behavior. It is postulated that in groups larvae can utilize warm areas and food with greater efficiency than independently. This efficiency, along with the stimulatory effect of social interactions, contributes to their rapid development. The shortness of the larval period, frees Bufo americanus from the necessity of breeding in permanent bodies of water, making it possible for them to spread into relatively dry regions. Tadpoles in rapidly evaporating pools survive by aggregating in depressions and aggregative behavior may provide some measure of protection against predators.