Swarming is a common mating behavior present throughout Diptera and, in particular, in species of lower flies (Nematocerous Diptera). Mating aggregations have been observed in the wild and in laboratory colonies of mosquitoes (Culicidae) and phantom midges (Chaoboridae), but have been assumed to be absent in their sister taxon, the frog-biting midges (Corethrellidae). Corethrellidae is a monogeneric family that includes over 100 species of frog-biting midges (Corethrella Coquillett spp.). In contrast to mosquitoes, female frog-biting midges find their host by eavesdropping on the mating calls of anurans to obtain a blood meal for egg development. Here we report the previously unknown mating swarms of frog- biting midges based on a laboratory study of Corethrella appendiculata Grabham. Contrary to previous speculations, we report that frog-biting midges aggregate in mating swarms. We thoroughly characterize such formations describing size and duration of the aggregation, sex ratio of the swarm, flight patterns of individual midges, influence of light intensity on the formation of swarms and use of swarm markers. In addition, we determine pairing and copulation patterns. Males and females copulate in venter to venter position. During the swarm, females and males meet in the air and most pairs fall together out of the aggregation onto the floor. We confirmed that virgin females are inseminated at the swarm excluding alternative hypotheses to explain the aggregations. Overall, we reveal the mating behavior of frog-biting midges for the first time filling critical gaps in information about the reproductive biology of this family.