Summary Observations on the swarming of the atroparvus variety of A. maculipennis are given, together with a description of the process of mating. Atroparvus swarms after sundown, beginning activity when light measures about 40 foot-candles, although temperature and probably humidity are also factors influencing time of swarming. The swarming occurs either inside buildings, in semienclosed pens, or in the open air. Most, but not all, females are fecundated while resting under shelter, without preliminary swarming of the male. Only a relatively few females enter the swarm. Hence it is suggested that swarming has largely lost its functional character, although the ritual is still preserved.