This paper describes the diet of the Red Knot (Calidris canutus) at Punta Rasa; the southern extreme of the Ramsar Site at Bahía Samborombón, in the Buenos Aires province, Argentina. Data presented here are mostly based on fecal analysis. We also used gizzard content analyses of four casualties that occurred during the catching operations. Remains of prey occurred in all droppings and consisted entirely of shell fragments of the Mud Snail (Littoridina australis, Hydrobiidae: Mesogasteropoda). A total of 295 droppings were analyzed containing Mud Snail fragments from which the size distribution of ingested L. australis could be estimated using a regression equation derived from a reference collection. In both study years (1995 and 2000), the size distribution of ingested Mud Snails differed significantly from size distribution present in the habitat; however, average length of ingested Mud Snails was similar in both periods. Three of the four gizzards analyzed contained Mud Snails. The fourth contained a common Scarabaeidae beetle (Dyscinetus sp., Coleoptera) present in the area in autumn. Results support the hypothesis that the Red Knot has a specialized diet and select medium-large size food items.