This paper describes the ultrastructural alterations observed in the tracheal epithelium of six sibling swine suffering from porcine immotile cilia syndrome (PICS) compared with those in human immotile cilia syndrome (HICS). As in some human cases, the tracheal epithelium of these pigs was lined by cilia-lacking cells. A variety of dynein defects in other pigs suffering from PICS have been previously observed. The spectrum of defects affords evidence that the PICS is genetically heterogeneous. Available data suggests that there are many similarities between HICS and PICS. Therefore, it is proposed that PICS may prove to be a useful animal model for the human disease.