Harbour Porpoises Phocoena phocoena were counted along a 270-km-long transect around southwestern Ireland on 17 July 1989, a day with exceptionally favourable weather conditions. A total of 251 porpoises were seen in a 1000-m-wide counting strip. After adjusting for porpoise movement, a mean density (± SE) of 0.77±0.26 animals per km2 was calculated for the continental shelf off SW Ireland. This allows the first provisional estimate to be made of the numbers of porpoises in the area. It is estimated that 19 210 (c.v.=0.34) animals were present in the shelf waters off SW Ireland, including 15% neonates.