The prevalence and intensity of Haemoproteus lari infestation were studied in a colony of Yellow-legged Gulls (Larus cachinnans) on the Medes Islands (NE Spain) during 1994 and 1995. Prevalence did not differ between years, sexes, clutch sizes or nesting habitats . More heavily infected females tended to lay smaller clutches and to be in a leaner body condition; in males, intensity of parasites was unrelated to their clutch size and their body condition. These results are partially consistent with the detrimental effect of blood parasites, but also with a tendency for stressed birds to relapse . Intensity of parasitemia differed both between years and among nesting habitats of the gullery, probably as a result of differences in bird quality at the time of sampling. We suggest that these effects should be controlled for in studies dealing with parasites in colonial nesting birds.