The characteristics of the food sources exploited by seabirds are one of the main factors determining the abundance and distribution of those birds. Determination of the diet over the year and between breeding and non-breeding seasons is important in understanding the food sources and their potential relationship with seabird population dynamics. We studied seasonality in the diet of the Rock Shag (Phalacrocorax magellanicus), a seabird endemic to southern Argentina and Chile, at a colony at Punta Loma, Chubut, by analysis of pellets (regurgitated casts) collected weekly during two consecutive non-breeding and breeding seasons in 2002 and 2003. We identified 19 prey-types in the diet. Polychaetes were the most common item, being present in 80–90% of the samples. Fish were also dominant in both non-breeding and breeding seasons (present in 60–80% of the pellets), especially species of Patagonotothen, Ribeiroclinus and Helcogrammoides. Crustaceans and cephalopods were also present in the diet, but to a lesser extent. There was a high overlap in dietary composition between non-breeding and breeding seasons, although more crustaceans and cephalopods were consumed during the non-breeding season in 2002. Results indicate that the food sources of Rock Shags are present in their feeding areas throughout the year, although many of these species are restricted in their availability, are of small size and have low energy content.