The marine otter Lontra felina lives along the Pacific coast of South America from 6°S to 56°S. A method for estimating marine otter abundance is described and a comparison of the characteristics of the terrestrial habitat of this species in southern Chile is made. From June 1999 to June 2000, we conducted eight hours of observations on one day each month at four study sites. An average of 3.8 observable otters/km were recorded, with significant differences between sites but no systematic trends with regard to season. Observable pups were recorded year-round. Otters spent 80% of their time out of view, engaged in behaviours other than feeding; observable otters were mostly seen feeding. The methodology described is effective for indexing abundances at small geographic scales. The marine otter needs both an aquatic habitat for foraging and a terrestrial habitat with an abundance of safe shelters, which, in turn, could limit the distribution of the species when human settlers and domestic dogs utilise the terrestrial habitats.