Many species of bats emerge from maternity colonies in a non-random mannerl however, quantifying the extent to which the emergence differs from random is complicated by the underlying variation in intensity of the emergence. In this paper an empirical method which overcomes these analytical difficulties is used to examine the functional nature of the pattern of bat emergence. The emergence behaviour from two large colonies of pipistrelle bats containing between 102 and 976 individual bats was monitored during the summers of 1992 and 1993. Both colonies had two primary exit holes with bats from one hole crossing the face of the second hole. Emergence events at the first hole triggered events at the second hole at both colonies demonstrating that the clumped emergence pattern was not a bottleneck artefact. The ectent of clumping was greatest during the first half of the emergence, when light levels were high, suggesting an anti-predation function. This pattern was not a consequence of a higher emergence intensity early in the evening.