Two sets of experiments were conducted to determine whether aggregation behaviour, commonly observed in tadpoles of the common toad, Bufo bufohad a predator avoidance function. In the first set of experiments, the distribution of toad tadpoles, of single or mixed sibship, was monitored in large artificial pools, one with fish chemical cues and one without. Aggregation behaviour was determined on the basis of the distribution of the tadpoles under these treatments by using two indices of cohesion: variance/mean ratio and a newly developed swarming index. The two indices were highly correlated. Tadpole groups were more cohesive (1) in the presence of fish chemical cues and (2) with other individuals from a single sibship. In the second set of experiments, tadpoles in different densities and distributions were presented to fish predators in a floating arena which allowed strike rate to be monitored but prevented the fish from capturing any tadpoles. Total strike rate per group increased with increasing group size, but strike rate per individual decreased, implying individuals gain from being in a larger group through dilution, but the group as a whole loses. These results are discussed in terms of selfish herd or cooperative group theories on the behaviour of aggregations.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour