Cayenne (Thalasseus sandvicensis eurygnathus) and Royal Terns (Thalasseus maximus) breed in mixed colonies in Argentina. This paper presents the first detailed information on their breeding diet and assesses differences and overlap between species in the type and size of prey. During 2004 and 2005, observations of prey delivered to mates and chicks were carried out at Punta León, Patagonia. The diet of Cayenne and Royal Terns comprised nine and 10 prey species respectively. Fish comprised >99% of the prey of both species of tern in the two study years. Both species had a fairly specialised diet based on pelagic schooling fish, mostly Argentine Anchovy (Engraulis anchoita) and two species of silversides (Odontesthes spp.). Despite observed overlap in trophic resources, our results showed that Argentine Anchovy was the main prey for Royal Terns, whereas the two species of silversides together with Anchovy comprised the bulk of the diet for Cayenne Terns. Fish delivered by Cayenne Terns were significantly smaller than those by Royal Terns. Results suggested that both type and size of prey may be important factors permitting food partitioning between the two species of tern during the breeding season.