An effective and inexpensive device is described for marking newly emerged black flies with fluorescent dust. Survival of marked adults of Simulium venustum s.l. and Stegopterna mutata s.l. did not differ significantly from unmarked individuals over a 5-day period. Marked individuals were attracted to human hosts in landing biting tests, and the portions of the self-marking trap lying just above the water surface did not appear to significantly hamper black fly emergence success.