The results of Butterworth et al. (1991) relating potential krill yield to a pre-exploitation survey estimate of krill biomass are extended as specified by the Third Meeting of Working Group on Krill. The most important of these extensions is integration over the ranges of uncertainty for a number of the model parameters. Results are provided for the probability of spawning biomass falling below various fractions of its median pre-exploitation level, as a function of the fraction of the biomass estimate which is set as the catch for a 20-year period. Three alternative fishing seasons are considered. Fishing in summer (near-coincident with the period of krill growth), or throughout the year, allows a greater catch (for the same risk of depletion) than in the case of fishing in winter.