<p>In sepsid flies, mating behavior is often observed at oviposition sites following oviposition, yet where and when virgin females mate remains largely undocumented. <i>Sepsis fulgens</i> is known to form large swarms away from oviposition substrates, previously hypothesized to be “hibernation phenomena.” Here, I report observations of three large swarms of <i>S. fulgens</i> in June and July in central Iran, where both mating attempts and copulations occurred. When provided with dung dishes near the swarm, gravid females left the swarm, oviposited in the dung, and adult flies emerged from it. Females captured in the swarms predominantly carried primordial or immature eggs, and their egg counts did not differ from those of the minority of swarm females with mature eggs. Additionally, when eggs were small, the ovaries lacked excessive tracheae that would indicate past oviposition. These observations suggest that swarm females were nulliparous. In contrast, females at oviposition and feeding sites (on fresh and old dung) mainly carried mature eggs and had lower total egg counts, consistent with prior oviposition. Laboratory-reared flies spontaneously released a lemony scent similar to that emitted by the swarms during their first week of adulthood, and it could also be induced by gently squeezing the abdomen, especially during the first two weeks. These findings challenge the hypothesis that <i>S. fulgens</i> swarms are related to hibernation. Although mating occurs at swarms, the rarity of copulations and gravid females suggests that mating is also not their only function. The swarms may serve multiple purposes, such as facilitating early mating and enhancing chemical defense via collective scent release.</p>