A detailed and comprehensive set of catch and effort data for the cod fisheries of 18 Norwegian bottom trawlers have been obtained for the period 1971ï¿½85, a period with few binding quota restrictions on vessel operations. Harvest functions have been designed and estimated. The independent variables are hours of trawling per vessel day and biomass of the cod stock (3+). Daily biomass estimates have been calculated by polynomial interpolation of the annual estimates of the International Council for the Exploration of the Sea (ICES). By maximizing the log-likelihood function using numerical methods, parameter estimates and performance indicators of the different models were obtained. The best result was obtained for a harvest model allowing for seasonal changes and with an autocorrelated error term. For this model, the stock-output elasticity is estimated at 0.424, the effort-output elasticity at 1.232, and the technological change at about a 2% annual increase in productivity. The seasonal changes in catchability are significant, with the lowest intra-annual catchability being less than 30% of the annual maximum.