Abstract.1The oviposition behaviour ofSimulium reptansL. is described from two sites on the River Spey, Scotland. Female aggregations were observed immediately downstream of oviposition sites and were composed mainly of gravid flies (range 60–80%0) together with smaller numbers that were either freshly bloodfed (range 2–17%) or infected with mermithid nematodes (range 0–30%).2The time from landing on the oviposition sites to the onset of oviposition was recorded. The time in the presence of greater than 1‐day‐old eggs did not significantly differ from sites with no eggs present. However, the presence of freshly laid or I‐day‐old eggs significantly shortened the time to onset of oviposition. The cues that elicit oviposition are unknown but it is speculated that they may involv