Trematodes and nematodes can be pathogenic helminths of birds. Every year during the breeding season, there is variable mortality among chicks from the largest Olrog's gull (Larus atlanticus) colony in Argentina. During two consecutive breeding seasons of Olrog's gull, we studied epidemiological and pathological aspects of infections by digeneans (Microphallidae) and nematodes (Acuariidae) in Olrog's gull chicks. Prevalence of nematode infection was 80.3% in 2005 and 89.2% in 2006, and mean intensity was 23.7 in 2005 and 50.8 in 2006. The risk for infection rose 34.3% and the intensity of infection 6.7% for every increase of 1 mm in head–beak length. The nematodes occupied the proventricular glands and caused disruption of their structure and mild inflammatory proventriculitis. Prevalence of digenean infection was 97.0% in 2005 and 97.3% in 2006. In 10-day-old live chicks, prevalence was 98.0% in 2006 and 95.3% in 2007. Infection was associated with severe catarrhal enteritis, lymphocyte/eosinophil-rich inflammatory responses, extensive fibroblast proliferation around the parasites, and disruption of the architecture of the adjacent crypts.