Chitinolytic activities, both chitinase (EC 3.2.1.14) and minimum chitobiase (beta-N-acetyl-D-glucosaminidase; EC 3.2.1.30), were measured in stomach and intestinal tissues and their contents, from 13 fish species. Higher activities were found in the tissues than in the gut contents, and higher activities were seen in the stomachs than in the intestines. Demersal species exhibited chitobiase activities very close to their chitinase activities, suggesting that these fishes can degrade chitin completely to its soluble, absorbable monomer, N-acetyl-glucosamine. This suggests that these species may catabolize chitin not just to penetrate prey exoskeletons but also to derive nutrients from the chitin itself. In contrast, three mesopelagic species exhibited low chitobiase but high chitinase activities. This chitobiase limitation correlated strongly with gastrointestinal tract morphology, with the myctophids having the greatest chitobiase limitation and the shortest alimentary tracts. The high chitinase activities measured in the myctophids reflect their ability to rapidly disrupt prey exoskeletons ingested during their nightly feeding in surface waters. Their chitobiase activities are greatly reduced because with rapid meal evacuation through a short gut there is little time for processing and limited energetic advantage in the complete degradation of chitin. These results suggest multiple roles for chitinolytic enzymes in marine fishes and that feeding habits and frequency may have a bearing on the evolution of their digestive enzymes systems.