The spatial and temporal Variation in the diet of Cormorants Phalacrocorax carbo visiting Switzerland in the non-breeding season was studied from 1985-1992, by means of regurgitated pellets and stomach contents. The results, combined with the data of most other dietary studies from Switzerland since 1974, rest on 4810 samples representing 24 122 fish from ten lakes and nine river stretches. Of 31 fish species occurring at the feeding sites, 23 were found in the diet, yet 5-7 accounted for 85-95% by numbers. Roach Rutilus rutilus was found in 58% of all samples and, together with Perch Perca fluviatilis, accounted for 65% by numbers. Diet was habitat-specific, and three main types could be discerned by canonical variate analysis: (1) diet strongly dominated by Roach and Perch, typical for most eutrophic lakes and empounded rivers; (2) diet containing a high percentage of whitefish Coregonus sp. and Tench Tinca tinca, on two lakes with extremely low Roach biomass; (3) diet dominated by Grayling Thymallus thymallus, Trout Salmo trutta or riverine cyprinids, in free-running rivers. Seasonal Variation was found to be small in Roach-dominated diets but strong at some other sites. Prey size ranged from small fry to adult fish of up to 800 g and was on average larger in rivers than in lakes. Small fish (< 12 cm) were mainly taken in lakes and reservoirs. At a landscape scale, the high proportion of Roach in the diet was associated with a preference for eutrophic lakes that support high densities of Roach and Perch. Roach was estimated to provide > 50% of the biomass ingested by Cormorants in Swiss waters. At a local scale, possible preferences were less conclusive, but Roach was apparently still overrepresented compared to abundance while whitefish was underrepresented. Roach and Perch are both shoaling species and are hunted by Cormorants which themselves congregate in large flocks. A similar preference for highly gregarious fish is apparent in most other studies from continental European inland sites.