This study compares the selectivity of 40 mm diamond and square mesh polyethyelene codends for size selectivity of two by-catch species, the European hake Merluccius merlucius and the striped red mullet Mullus surmuletus commonly captured in the beam trawl fishery in the Sea of Mannara. Data were collected using a conventional rigged twin beam trawl between 26 March and 20 April 2007. Covered codend method was used to obtain the selectivity data which analyzed by means of a logistic equation with the maximum likelihood method. The pooled length at 50% retention, L was found to increase with squre mesh shape for both species. L values of 40 mm diamond and square mesh codends were 9.8 and 13.2 cm for striped red mullet, respectively. 40 mm diamond mesh codend is not appropriate for the Minimum Landing Size (MLS) of 11 cm for the striped red mullet. Both the diamond and square mesh codends also retain a significant amount of undersized hake compared to the MLS of 25 cm. Only 40 mm square mesh codend provides satisfactory selection for striped red mullet in the rigged beam trawl fishery of the Sea of Mannara.