Abundance of Commerson s dolphin, Cephalorhynchus commersonii (Lacépède 1804), in the eastern portion of the Strait of Magellan was estimated by means of aerial transects in autumn (June 1996). Using a helicopter at 100 m altitude and 129 km h-1, 58 sightings representing 98 animals (average group size Es = 1.7, SD = 0.13) along 818.6 km of linear transects were recorded. Data analysis with "Distance" software gave a density estimation of 0.34 ind km-2 (CV = 0.27) and a population size of 1,206 (95% CI 711-2,049 individuals). Commerson s dolphin showed a habitat preference for the narrows, compared to open waters in the study area.