Seventy-eight harbour porpoises, Phocoena phocoena (33 females, 45 males), were obtained in summer (June-September) as incidental by-catch from the cod fishery in the Gulf of St. Lawrence and examined for the presence of cranial sinus nematodes. Stenurus minor (Kühn, 1829) Baylis and Daubney, 1925 were present in the cranial sinuses of all adult porpoises (>1 year old, N = 66, mean intensity = 2362, range = 87-8920) and absent in all young of the year (<1 year old, N = 12). Only fifth-stage worms were observed and these were equally distributed between the right and left sides of the skull (mean intensity = 1158 and 1213 in the left and right side, respectively). However, S. minor were approximately twice as numerous in the frontal sinuses as in the ear sinuses. Mean intensities of S. minor were similar among all infected porpoises. Parasite load had no apparent effect on porpoise body condition (measured as percent blubber mass of the carcass). No gross lesions associated with the presence of numerous S. minor in the cranial sinuses were observed. There was an inverse relationship between the intensity of S. minor infection and mean worm length, which is suggestive of a "crowding effect." Mean worm length was 17.8 ± 0.2 mm in lightly infected porpoises and 16.1 ± 0.2 mm in heavily infected animals.