Demersal seining is an important active fishing method to harvest cod (Gadus morhua) in Norwegian fisheries. Knowledge about size selectivity of cod in this type of fishing gear is therefore of importance for managing the exploitation of cod resources. However, very limited data exists on the size selection of cod in the square mesh codends mostly applied in this fishery. By using the fish morphology and computer-based simulation method FISHSELECT, we investigated the size selection of cod in square mesh codends for demersal seining. We were able to explain and understand existing experimental selectivity results and predict the effect of design changes in the codend. The results showed that the currently applied codend designs are adequate to ensure low catches of cod below the minimum size for this fishery, but they also indicated that a considerable part of the size selection may occur through slack meshes. Thus, it is likely that at least part of the codend mesh selection may occur during the process of bringing the catch on board, when the gear is at the surface.