One of the primary aims of FIBEX (First International BIOMASS Experiment), 1981 was to study the methodology for assessing the abundance of krill. The survey design used in the southwest Atlantic study area of the FIBEX is described in this paper. Sampling involved the use of echo sounders for estimating krill abundance as well as collection of data on the size, density and distribution of krill swarms. In addition, information on surface water temperature, salinity and fluorescence as well as on seabirds was also collected. The study area was subdivided into several geographically distinct subareas in each of which randomly spaced transects were located. Subareas were treated as strata and a stratified random sampling method was used. The survey was done in two phases. In the first phase a fairly evenly dispersed subsample of transects was surveyed and these were also used to fix stratum boundaries. In the second phase the remaining transects were surveyed, using the stratum boundaries defined from the first phase. The design of the survey was directly related to the subsequent method of data analyses, some main aspects of which are discussed. The analytical formulae for the analyses are also presented