During 1980–97, trichomoniasis was detected in nestlings of Bonelli's eagle Hieraaetus fasciatus in Catalonia (Spain). In 1993 Trichomonas gallinae was isolated in 36% of nestlings (n = 39) and affected 41% of broods (n = 22). Overall, trichomoniasis was one of the most important single nestling mortality factor, accounting for 22% of total chick mortality, and causing the death of 2% of chicks. Trichomoniasis deaths took place during the second half of the nestling period. The median age at death was 45.5 days. Although the presence of the parasite was not related to the composition of the diet or parental age, pairs that developed the disease ate more pigeons and included more often non-adult birds. At present trichomoniasis apparently has little demographic impact on the Bonelli's eagle population in Catalonia, but the eventual spread of this disease in chicks and its unknown effects on adults might be of concern.