We report on the incidence of kleptoparasites and the host avoidance responses in a colony of the Sandwich Tern (Sterna sandvicensis) at l’Albufera de Valencia, eastern Spain, in 2004. Apart from conspecifics, five species were kleptoparasites of breeding Sandwich Terns, particularly Black-headed Gull (Larus ridibundus) and Slender-billed Gull (L. genei). Avian kleptoparasites attacked 24% of the Sandwich Terns returning to the colony with fish (N = 659) and robbed 5.3% of them. Most terns (85%) delivered their prey to a mate or chick. Fish carried by terns were mainly Clupeiformes species (96%). Prey size declined significantly as the season progressed. The probability of attack increased significantly with fish size and declined significantly as the season progressed. Most attacks were on terns as they landed to deliver their prey. Overall success rate of attacks was 12.5% and were mostly performed by single birds. The time that the terns spent flying before landing to deliver prey increased with the size of carried fish.