This paper examines trends in abundance and distribution of harbour porpoises in Swedish waters. Returns from questionnaires suggested a decline in number of sightings per year in all areas around the Swedish coast between the 1950s and 1980s. A network of observers was organised in 1988. A comparison of observations made by the network between 1988 and 1989 showed a decline in mean number of sightings per observation day. The results presented indicate that harbour porpoises have declined in Swedish waters during the 1960s and 1970s and continue to decline in the late 1980s. Few sightings have been reported from the Baltic Sea since the 1960s. Dedicated surveys were made during both 1988 and 1989 by sailboat and aeroplane. Few harbour porpoises were sighted during these surveys. Results show that aerial surveys are a preferable survey method.