Snow crab (Chionoecetes opilio) and Tanner crab (C. bairdi) fisheries in the eastern Bering Sea are managed by using swept-area estimates of biomass based on an annual survey conducted with a 83-112 eastern bottom trawl. Estimates of net efficiency (i.e. the capture probability of crab that occur between the wing-tips of the trawl net) are needed to correct the biomass estimates for any size selectivity by the trawl. Data on net efficiency were obtained experimentally by attaching an auxiliary net beneath the trawl net to capture crab escaping under the trawl footrope. Net efficiency is then the quotient of the trawl catch divided by the combined catch of the trawl and auxiliary nets. Mathematical models of the relationship between net efficiency and carapace width were formulated and fitted to the experimental data. Net efficiency for both species first decreased with increasing carapace width until a minimum efficiency was reached near 50 mm carapace width. At larger sizes, efficiency increased asymptotically with carapace width. Net efficiency for mature female Tanner crab was lower (0.47) than for males and immature females combined (0.72) at the same mean carapace width as mature females (66-107 mm). Net efficiency did not differ between morphologically mature and immature male Tanner crabs of the same carapace width.