Diel variation in feeding rates of Eudiaptomus graciloides was measured in situ with the gut fluorescence method during May and June in the eutrophic Frederiksborg Slotssø Pronounced diel periodicity occurred in both periods with highest feeding rates during the night. The food concentration as well as the periodicity was almost constant with depth, indicating that the feeding activity represents a behavioral periodicity. The gut evacuation measured in the laboratory on field-collected copepods was exponential and temperature-dependent. The exponential decline in gut content was found for both copepods feeding continuously and when feeding had ceased. Considering spatial and temporal variation in feeding activity, daily ratios were 61-68° body carbon per day, and higher than if calculated from existing laboratory data of short-term feeding dates.