SUMMARY.1. Weight‐specific respiration and feeding rates ofGammarus pseudolimnaeuswere inversely related to body weight at 12°C, the temperature of the stream from which they were collected.2. At this temperature animals could digest about a third of ingested, purified14C‐cellulose. In animals greater than 20 mg live weight, about 38% of ingested cellulose was metabolized; animals smaller than 20 mg metabolized about 28%.3. Respired CO2from cellulose metabolism was 33–47% of measured uptake of C2in animals greater than 20 mg live weight and approximately 25% in smaller animals.4. Ability to digest cellulose would be of nutritional benefit toGammarusfeeding on any plant material, but would be of particular advantage in the spring/summer period when fine detritus is the main foo