The present study describes the occurrence of lungworm infections in stranded beluga (Delphinapterus leucas) from the St. Lawrence Estuary (Quebec, Canada). Estimated intensities of nematodes were obtained from systematic analysis of both lungs of 33 carcasses sampled from 2004 and 2013. Associations between nematode abundance, sex, age groups, body condition, and cause of death were evaluated. Lungworms were detected in 91% of the cases. Verminous pneumonia was identified as the cause of death in five beluga. Two species of pulmonary nematodes were identified: Stenurus arctomarinus and Halocercus monoceris. Nematodes of an unidentified species were also present in one beluga. Coinfections with S. arctomarinus and H. monoceris were present in most of the infected beluga (63%). Halocercus monoceris was the most common species of nematode found with a prevalence of 85% (mean intensity: 254 nematodes; min–max: 1–1931; median: 60). The prevalence of S. arctomarinus was 61% with a mean intensity of 34 nematodes (min–max: 1–356; median: 2). The mean abundance of H. monoceris was significantly higher in animals for which verminous pneumonia was the cause of death. Newborns were significantly less infected by both nematodes (S. arctomarinus prevalence: 20%, mean intensity: 2; H. monoceris prevalence: 40%, mean intensity: 4.5) than weaned juveniles and adults (S. arctomarinus prevalence: 68%, mean intensity: 36; H. monoceris prevalence: 93%, mean intensity: 273). This suggests that even if vertical (transplacental or transmammary) transmission occurs, this mode of infection may be less significant than horizontal transmission.