During field studies of the behaviour of larvae in colonies of Malacosoma disstria Hbn., it was observed that two species of ants, Camponotus herculeanus ligniperdus (Latr.) and Formica fusca L., were common on the twigs of the small trees, Populus tremuloides Michx., on which the caterpillars were established. Ant nests were common near the bases of trees, and the worker ants climbed the trunks and moved along the branches either to forage or to tend aphid colonies. When the tent caterpillars were small, they were frequently attacked by the ants. There were two distinct types of attack: one the result of foraging by worker ants and the other the result of defence of aphid colonies by the ants.