Swarming behaviour of male braconid wasps, mostly of the genus Bracon and, in more detail, of the genus Blacus, is described. Blacus ruficornis Nees, a species that starts swarming later than the closely related B. maculipes Wesm., supplants the latter species from the habitat where swarming occurs. Females are present in the surrounding area but their occurrence in the swarms is occasional. The hypothesis is put forward that the aims of the swarming are 1) increasing of the low population density within given territory where males subsequently look for mates, 2) providing information on the population density and sex ratio by means of the feedback mechanism, and 3) physiological stimulation of males. none