To determine what capabilities wood-eating and detritivorous catWshes have for the digestion of refractory polysaccharides with the aid of an endosymbiotic microbial community, the pH, redox potentials, concentrations of short-chain fatty acids (SCFAs), and the activity levels of 14 digestive enzymes were measured along the gastrointestinal (GI) tracts of three wood-eating taxa (Panaque cf. nigrolineatus "Marañon", Panaque nocturnus, and Hypostomus pyrineusi) and one detritivorous species (Pterygoplichthys disjunctivus) from the family Loricariidae. Negative redox potentials (¡600 mV) were observed in the intestinal Xuids of the Wsh, suggesting that fermentative digestion was possible. However, SCFA concentrations were low (<3 mM in any intestinal region), indicating that little GI fermentation occurs in the Wshes' GI tracts. Cellulase and xylanase activities were low (<0.03 U g ¡1 ), and generally decreased distally in the intestine, whereas amylolytic and laminarinase activities were Wve and two orders of magnitude greater, respectively, than cellulase and xylanase activities, suggesting that the Wsh more readily digest soluble polysaccharides. Furthermore, the Michaelis-Menten constants (K m ) of the Wshes' -glucosidase and N-acetyl--D-glucosaminidase enzymes were signiWcantly lower than the K m values of microbial enzymes ingested with their food, further suggesting that the Wsh eYciently digest soluble components of their detrital diet rather than refractory polysaccharides. Coupled with rapid gut transit and poor cellulose digestibility, the wood-eating catWshes appear to be detritivores reliant on endogenous digestive mechanisms, as are other loricariid catWshes. This stands in contrast to truly "xylivorous" taxa (e.g., beavers, termites), which are reliant on an endosymbiotic community of microorganisms to digest refractory polysaccharides.