Filtering and feeding rates of Daphnia middendorffiana at Point Barrow, Alaska were measured as a function of temperature, food density, animal length, and time of day.The functional relationship between feeding rate and food density followed the hyperbolic pattern typical for filter feeders. Daphnia middendorffiana was found to have comparatively high filtering capabilities at its temperature optimum (around 11?C), with filtering rate increasing with length according to the equation F?=.458L3.17. The exponent of this equation was found to be a function of temperature, and the coefficient a function of food density.A bimodal diel rhythm in feeding rate was found in this species in experiments where the animals were held under constant conditions. Feeding rates were maximum at 1400 and 2400, the time of day when the temperature cycle in the ponds passes through the mean daily temperature, which is optimum for feeding in D. middendorffiana. Thus the timing of the rhythm may serve to maximize the daily food gathering ability of this species.