Methods for calculating the horizontal opening (and, thus, swept area) of a bottom trawl are discussed. In situ measurements of the FP-120 trawl used during the 1993/94 UK fish stock assessment survey around South Georgia on the MV Cordella are presented. Regression equations for the estimation of horizontal opening from other, more easily measured variables, such as tow speed and trawl depth, are proposed. Biomass estimates for the major fish species around South Georgia are estimated using a new regression equation. The results of this calculation are compared to values obtained from a previous equation. The resulting difference in biomass estimates is of the order of a 10 to 14% increase, depending upon species.