Nigronia serricornis (Dark Fishfly) males were observed patrolling the borders of a small stream in northern Virginia. They flew toward females perched low in streamside vegetation. The first male to reach a female mated with her. Later arrivals left after a brief inspection of the mating pair. The mating system of this species appears to be based on nonaggressive scramble competition for widely scattered receptive females. Copulation in this member of the Corydalidae was prolonged.