A series of computer simulations were carried out to predict how the selectivity of haddock in a diamond mesh cod-end varies according to total catch, by-catch, entry time of by-catch and the shape of the catch build-up in the cod-end. Results were compared to those from sea trials. I also investigated the predicted selectivity on the assumption that the shape of the cod-end does not change as the catch accumulates. The latter investigations were carried out for various shapes of the cod-end. The simulations indicated that the 50% retention length (L50) increases with both the total catch and the by-catch weight. They also indicated that the entry time of the by-catch may have an important influence on both the 50% retention length and selection range (SR). The shape of the catch in the cod-end had a similar effect, for the cases analyzed.Different cod-end shapes showed very large differences in the values for L50 as well as more narrow SRs, assuming that the shape of the cod-end remained constant during the tow. The simulations strongly indicate that the change of shape of the diamond mesh cod-end as the catch builds up during towing is a major contribution to the SR.