We studied 36 dogs (Canis familiaris) from the Can Roqueta site in the Catalan pre-littoral depression (Barcelona), dated between the Late Bronze Age and the First Iron Age (1300 and 550 cal BC). We used a sample of 27 specimens to analyse the evolution of the dogs' diet based on the carbon delta C-13 and nitrogen delta N-15 isotope composition. The results show a marked human influence in that these natural carnivores display a highly plant-based diet. The offset between canids and herbivorous ungulates does not reach the minimum established for a trophic level, which implies an input of C-3 and C-4 (millet) cultivated plants. Moreover, the homogeneity in the values indicates that humans prepared their dogs' food.