Red mullet (Mullus barbatus) is one of the most important commercial species for the multi-species Mediterranean demersal trawl fisheries. Several experimental studies have been carried out to assess the codend size selectivity for red mullet in the last two decades. However, while the number of experiment-based selectivity studies on red mullet are considerable, very little theoretical work exists which tries to understand and explain the size selectivity for this species in trawl codends. Therefore the main objective of this study was to establish a theoretical framework to explain and predict the size selection of red mullet in bottom trawl codends of different mesh size and configuration. We identified the important morphological characteristics of red mullet which determine the size selection in towed fishing gears using the FISHSELECT methodology and further used this methodology to predict size selection for a wide range of mesh configurations through simulations. For example we predicted the 50% retention length for red mullet to be in the range 9.2-15.8 cm for the legal 50 mm mesh size diamond mesh codend and in the range 10.6-14.2 cm for the legal 40 mm mesh size square mesh codend. The simulated size selection was found to agree well with previously published results for codend size selection of red mullet obtained from sea trials. Thus based on fish morphology we were able to understand and explain previous results leading to a better understanding of the codend size selection for this species. Further the morphologically based results enable predictions of size selection of red mullet in codend designs not previously tested experimentally. (C) 2015 Elsevier B.V. All rights reserved.