The utility of square mesh windows as by-catch excluders when placed either in the trawl upper belly or at the top of the cod-end is examined for a number of species captured off the Portuguese south coast. Data were obtained for the blue whiting, Micromesistius poutassou (Risso, 1826), the boarfish, Capros aper (L.), the horse mackerel, Trachurus trachurus (L.), the blue jack mackerel, Trachurus picturatus (Bowdich, 1825), the chub mackerel, Scomber japonicus Houttuyn, 1782, the European hake, Merluccius merluccius (L.) and the rose shrimp, Parapenaeus longirostris (Lucas, 1846). Active escape behaviour was evidenced for blue whiting and blue jack mackerel, these being the only species escaping in significant amounts, particularly when the square mesh window was placed on the top of cod-end (54 and 48%, in weight, respectively). For the remaining fish species, the data suggest the need for appropriate stimuli in order to improve escape behaviour. The losses of shrimps were equal for both window locations (about 11%, in weight).