The Joaquín Costa reservoir contains a mixed fish assemblage of native and exotic species. Feeding habits and feeding relationships of species in the fish assemblage were analysed over a one year cycle. Differences in diet composition were found both between species and within species among seasons. Food overlap and trophic similarity among species also showed seasonal variations. Cluster analysis differentiated four groups of predominant diet: (1) macroinvertebrates (trout and largemouth bass), (2) detritus (nase), (3) cladoceran crustaceans and (4) an omnivorous feeding regime, with large seasonal variations in food habits. Food of fish species included in groups 3 and 4 (roach, white bream, barbel, common and mirror carp) varied seasonally. Using graphical models of feeding strategies, similarity indexes, cluster and multivariate analyses based on the relative importance of food categories in the diet of the species, we illustrate that the fish assemblage showed food resource partitioning according to food habits and foraging habitats within the reservoir.