The objective of this study was to characterize pathologic changes associated with experimental infection of harp seals (Phoca groenlandica) with the lungworm Otostrongylus circumlitus (Metastrongyloidea: Crenosomatidae). The leukocyte differential cell count in samples obtained by unguided bronchoalveolar lavage (BAL) and the intensity of the histologic lesions in the lungs were assessed in seven harp seals experimentally exposed to 300 infective, third-stage O. circumlitus larvae. Seven unexposed harp seals were used as controls. First-stage larvae were observed in the feces of three of the seven exposed seals at 38, 42, and 45 days postexposure (dpe). Adult nematodes were found in the right primary bronchi of two of these three seals at necropsy 53 dpe. Fifty-six BALs were performed on the 14 seals. No statistical difference was observed between the exposed and control seals and among the four sampling times in percentage of neutrophils and macrophages in the BAL fluid. A significant difference was observed between the exposed and control seal groups in the percentage of eosinophils (P<0.0001), the count of eosinophils having increased by a factor of 70.4 in exposed seals. Significant statistical differences were observed between exposed and control seals in intensity of interstitial inflammation (P=0.001), bronchitis (P=0.02), bronchiolitis (P=0.04), alveolitis (P=0.03), and interstitial granulomatous inflammation (P=0.04). Our findings showed that harp seals are susceptible to infection with O. circumlitus. However, parasitic infections were transient and of low intensity, at least under our experimental conditions.