Deciduous leaves provide a major allochthonous source of energy to stream food webs, particularly in small, shaded headwater streams. To digest cellulose and other structural carbohydrates in plants, invertebrates must use enzymes that degrade these polymers. Our research focused on the digestion of cellulose by the freshwater amphipod Gammarus lacustris. We used a feeding experiment and laboratory enzyme assays to investigate 3 potential sources of cellulase in G. lacustris: 1) tissue-level production of cellulase by G. lacustris, 2) cellulase activity by gut endosymbionts living in G. lacustris, and 3) ingestion of fungal cellulase exoenzymes that remain active after ingestion by G. lacustris. We determined that G. lacustris possessed endoglucanase and β–glucosidase enzymes that are active on soluble cellulose derivatives. We did not detect cellobiohydrolase activity toward crystalline cellulose in G. lacustris. Endoglucanase and β–glucosidase activity appeared to originate from amphipod tissues; in this experiment microbial enzymes did not contribute to cellulase activity. These results are consistent with other studies indicating that many invertebrates produce endoglucanases and β–glucosidases but very few, if any, produce cellobiohydrolase endogenously.