We studied the surface and dive times of finless porpoises (Neophocaena phocaenoides) in Hong Kong from three land-based survey sites in winter/spring 2000, to allow the estimation of the trackline detection probability [g(0)] for helicopter line transect surveys in Hong Kong. We determined that groups of finless porpoises in Hong Kong spend on average about 60% of their time at or near the surface, and 40% of their time on a long dive (30 sec. or greater). Surface times apparently increase in larger groups. We conclude that finless porpoises in Hong Kong, while being challenging targets for sighting surveys, are apparently available at the surface to be seen for a higher proportion of time than are harbour porpoises in some deeper water habitats.