Olrog's Gull Larus atlanticus is an endemic species of the Atlantic coast of southern South America. It is one of six globally threatened gull species in the world, and is classified as Vulnerable. We studied age-related differences in the diet and foraging behaviour of this species in Mar Chiquita Lagoon during the non-breeding season. Diet was assessed by analysis of regurgitation casts, prey carcasses and direct observation, while foraging behaviour was quantified by focal observations. Grapsid crabs were by far the main prey in frequency of occurrence, followed by fish and snails. Some differences in diet were found between ages, particularly in spring when mature birds start to feed on the eggs of ovigerous females of the crab Cyrtograpsus angulatus. Foraging behaviour was similar between ages which is attributed to large resource availability, foraging strategies and social factors.