Bradysia impatiens(Johannsen) females emit a sex pheromone that elicits male upwind flight in the male, evidently via a mechanism of optomotor-modulated anemotaxis. Maximal male wing fanning and upwind walking to female body extract occurs between 5 h before to one h after the initiation of scotophase on a 16:8 LD. Mated and unmated females appeared to have similar quantities of extractable pheromone. The mating sequence appears relatively stereotyped.