Shrimp trawling in Palk Bay, Sri Lanka, is conducted in a 9-m mechanized craft with 30-hp inboard engine. Penaeus semisulcalus is the major species. .An experiment consisting of twenty trawl operations employing the covered codend technique was carried out using 25, 30, and 40-mm codends in tits' fish-cum-shrimp trawl nets for mesh selectivity studies..The codend with the 25-mm mesh size hardly released any, whereas others released a fair number of commercially valuable P. semisulcalus. The 50% retention lengths for P. semisulcalus were 1.96 cm and 2.27 cm for the 30-mm and the 40-mm codend, respectively. When compared with the traditional trawls, the new fish-cum-shrimp trawl with larger mesh size performed better by bringing a slightly larger amount of shrimp and a greater quantity of fish. The instantaneous fishing mortality values during January to June 1984 ranged from 0.16 to 0.38 for P. semisulcalus and 0.09 to 0.37 for the bycatch. Yield-per- recruit analysis indicated no immediate threat from the new trawl to the shrimp resources in Palk Bay, but rather a beneficial effect because of the introduction of larger mesh size in the codend.