Abstract There was a progressive increase in the size of the aerobic heterotrophic bacterial populations along the gastro-intestinal tract of farmed Dover sole. Moreover, higher counts were recorded in juvenile than in adult animals. Thus, in juvenile fish, 5.2 × 105, 8.0 × 105 and 9.8 × 106 aerobic heterotrophs/g were recovered from the stomach/foregut, midgut and hindgut/rectum, respectively. In adult fish, comparative samples revealed the presence of only 3.0 × 104, 7.0 × 104 and 2.3 × 105 bacteria/g, respectively. There bacteria were equated with Acinetobacter, Alcaligenes, Enterobacteriaceae representatives, Flavobacterium, Micrococcus, Photobacterium, Staphylococcus and Vibrio. Of the compounds tested, many isolates, particularly those recovered from the hindgut/rectum, degraded p-nitrophenyl-β-N-acetylglucosaminide, chitin and collagen. Consequently, it is likely that such organisms may contribute to nutritional processes within Dover sole.