Abstract An analysis of 38 bottom trawl hauls at depths of 251–737 m off the Balearic Islands (western Mediterranean) during autumn 2002 and spring 2003 was used to compare the species and size selectivity of 40-mm diamond- and square-mesh codends under commercial conditions. There was no difference in the catch composition or the yield that could be attributable to mesh shape, although the percentage of total and commercial species discarded with a diamond mesh was higher than with a square mesh. At least in the short term, the escapement ratio and the economic loss with the diamond mesh were lower than with square mesh, but economic efficiency was no different between them. For all the main species compared, except one flatfish, size selectivity parameters were lower for the diamond-than for the square-mesh codend. Selectivity values for the one flatfish species were similar. From the results it is concluded that, within the context of precautionary management, introduction of a 40-mm square mesh in the codend could be an appropriate and plausible measure to improve the state of the resources exploited by the deepwater crustacean trawl fishery of the upper slope off the Balearic Islands, and to reduce the impact of the fishery on the ecosystem.