Nematodes of the genus Serratospiculum are common and usually innocuous inhabitants of the air sacs of several species of falcons. However, the abdominal and thoracic air sacs of a prairie falcon (Falco mexicanus) that died in respiratory distress were filled with hundreds of adult parasites. Illnesses in a peregrine (Falco peregrinus tundrius) and another prairie falcon, tentatively diagnosed as serratospiculiasis, were successfully treated with thiabendazole given orally. Embryonated ova were found in the feces of 8 of 73 falcons representing five species.