Demersal seining is an important 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, limited data exist on the size selection of cod in the square mesh codends mostly applied in this fishery. By using knowledge of fish morphology and the computer-based simulation method FISHSELECT, we investigated the potential for 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 part of the codend mesh selection may occur when the gear is at the surface.