As profit margins decline, and gains from husbandry improvement diminish, marine fish farmers around the Mediterranean are considering the development and use of genetically improved fish strains to increase productivity and profitability. To date, published data on the use of crossbreeding (intraspecific hybridization) and selection in marine Mediterranean cultured species are available only for the gilthead seabream (Sparus aurata). Crossbreeding: to date, crosses between S. aurata strains usually yielded little heterosis. Between strain selection: differences for growth and survival performance in culture were detected among several Eastern Mediterranean strains of gilthead sea bream S. aurata. Within strain selection: mass selection rather than family selection, was technically possible for the group spawning S. aurata. Single generation up-selection for weight increase yielded -0.4 per cent to 10.9 per cent gain (average gain of 7.3 per cent) and down-selection for weight decrease yielded 8.5 per cent to 16.2 per cent losses at harvest weight. Gain from selection for increase weight was attributed to increased food conversion efficiency.