The harbour porpoise Phocoena phocoena is by far the most common cetacean in the North Sea. It was considered common in the southern part of the North Sea in the first half of the 20th century but since the 1960ies numbers gradually decreased. In the 1970ies and 1980ies the species probably was virtually absent from Belgian waters. Since the early 1990ies the abundance of the porpoise has increased markedly in the coastal waters of Belgium and The Netherlands. During the last years, especially between January and April, frequent sightings were made in Belgian waters, even from the coast. The higher number of sightings is reflected in the number of strandings and bycatches. While in the 1980ies and early 1990ies, only 3 to 6 porpoises washed ashore each year, this number gradually increased, and in 2003 and 2004 respectively 37 and 41 porpoises were found on Belgian beaches. Autopsies were performed on all stranded animals, and especially in 2004 bycatch in fishing gear was identified as a major cause of death. Particularly gill net fisheries, otherwise considered as a relatively environmentally friendly fishing technique, is known to incidentally kill thousands of porpoises each year in the North Sea. In Belgium a large proportion of the incidental catches had undoubtedly occurred in recreational beach gillnet fisheries. Several explanations for the increased number of porpoises in the southern North Sea are put forward. It is unlikely that the higher numbers are the consequence of a growing population size. We believe that in recent years a change in the migration patterns has occurred. This was possibly caused by changed environmental conditions, causing a change in the feeding opportunities for this species in the North Sea.