Aggregations of tadpoles of the American toad, Bufo americanus are described and a scheme for classifying them is proposed. Aggregations are recognized using two major criteria, tadpole density and behavior. Aggregated tadpoles engage in two types of social interactions, butting and coordinated swimming. Butting occurs as larvae swim about and butt and push against each other. In coordinated swimming, tadpoles move in the same direction and roughly parallel to each other. Aggregations are classified into two broad categories: Stationary Aggregations, which do not change position, and Moving Aggregations, which flow as a unit from place to place. Necrophagous Groups are Stationary Aggregations in which tadpoles vigorously feed from a dead tadpole or other carrion. They are especially common when larvae are crowded into small, shallow ponds. Feeding Groups are Stationary Aggregations of feeding tadpoles (Gosner stages 25-41) feeding on periphyton or from the pond bottom. A third type of Stationary Aggregation, the Metamorphic Group, is made up of tadpoles which have nearly completed metamorphosis (stages 42-45) and no longer feed. They occur in shallow water near shore and transforming individuals emerge from them and move onto land. There are two kinds of Moving Aggregations, Streams and Schools. Tadpoles in Streams swim together in moderately dense processions and feed rather infrequently. Schools are made up of tadpoles in closer contact that are primarily feeding as the group advances. Aggregations result from a complex of interacting factors including light intensity, water temperatures, type and distribution of food, and social interactions among the larvae themselves. Many of the aggregations result in a more efficient utilization of food resources.