E. civile is a nonterritorial damselfly whose males exhibit contact-guarding, and the tandem-pairs oviposit alone or in small groups. Three possible factors influencing the formation of aggregations were investigated; single-male interference, female oviposition efficiency and predation risk. Although aggregating reduced the risk of interference and may lower predation risk, for this species, oviposition efficiency was reduced under crowded conditions. This reduction in efficiency was primarily due to the incidental contact that occurred among ovipositing pairs. Because contact-guarding was effective at preventing takeovers, and males had few mating opportunities on a given day, males as well as females should prefer conditions favoring high oviposition efficiency. Future research on oviposition behaviour should include a measure of oviposition efficiency, because it can be an important factor influencing site selection.