The drag (a force imparted into the mean direction of flow on a submerged object by fluid flowing around it) on single larvae of the black fly Simulium vittatum Zett. in a thin, growing boundary layer was measured using a modified analogue ammeter connected to a variable voltage power supply. The average drag on a single larva of S. vittatum ranged from 2.70× 10 -5 Newtons at 10 cm/s to 4.24× 10 -4 N at 90 cm/s. The drag was similar whether the larva was attached to an ovoid wire or to a piece of cover slip. Drag on larvae with abducted labral fans was as much as two times the drag on larvae with adducted fans. Drag on larvae that had bent their bodies to probe the substrate was higher than drag on filter-feeding larvae. The mean (±standard error) strength of a dead larva's attachment to its silk pad was 1.2× 10 -2 ± 2.4× 10 -3 N, with a range of 4.9× 10 -3 N to 2.8× 10 -2 N. It is concluded that drag is not strong enough to pull a larva off the s...