Primary ciliary dyskinesia (PCD) is a genetically and phenotypically heterogeneous disorder, characterized by progressive development of bronchiectasis, inflammation, and features characteristic of chronic obstructive pulmonary disease. We report here that a murine mutation of the evolutionarily conserved adenylate kinase 7 (Ak7) gene results in animals presenting with pathological signs characteristic of PCD, including ultrastructural ciliary defects and decreased ciliary beat frequency in respiratory epithelium. The mutation is associated with hydrocephalus, abnormal spermatogenesis, mucus accumulation in paranasal passages, and a dramatic respiratory pathology upon allergen challenge. Ak7 appears to be a marker for cilia with (9 + 2) microtubular organization. This is suggested by its tissue specificity of expression and also the stringent conservation of Ak7 ortholog structure only in protozoans and metazoans possessing motile (9 + 2) cilia. Collectively, our results indicate an ancestral and crucial role of Ak7 in maintaining ciliary structure and function, and suggest that mutations of the human ortholog may underlie a subset of genetically uncharacterized PCD cases.