Energy densities and percent dry matter of zooplankton were estimated from both field collections and from stomach contents of rainbow trout Oncorhynchus mykiss. Dry-mass energy densities were similar for zooplankton collected from the field and from fish stomachs, averaging 22.7 kJ/g. The percent dry matter of zooplankton from fish stomachs was approximately twice that of zooplankton from field collections, although species composition was similar. Energy density of wet zooplankton was consequently twice as high in fish stomachs as it was in field collections. These results indicate that the wet-mass energy density of zooplankton prey used in fish bioenergetics models should be increased over values that have been reported in the literature.