With the United Kingdom required to designate Special Areas of Conservation (SACs) under Natura 2000 by 2012, it is important to understand site-specific activity and habitat use in order to identify potential sites. Shore-based observations of harbour porpoises (Phocoena phocoena) were carried out from two sites in North Devon, UK. Morte Point was surveyed during August and September 2001 and Lee Bay was observed during August and September 2002. Focal group follows were conducted to monitor porpoise behaviour and movement over tidal and diurnal cycles. At Morte Point, porpoises were found to aggregate in an area of high tidal flow, where prey items are likely to be abundant. While no differences were observed in occurrence during diurnal and tidal cycles, group size and distance from shore were found to be statistically significant with time of day at Morte Point. Porpoises were observed feeding here 59.9% of the time, with 78.0% of feeding taking place in multi-species associations and larger group sizes being observed at this site. At Lee Bay, porpoises were found to utilise an area of high heterogeneity, where rocky outcrops divide an otherwise sandy bay. In contrast to Morte Point, porpoises were observed feeding at Lee Bay 27.6% of the time, spending 34.7% of the time engaged in travelling in smaller groups. Despite these differences, behaviour and group size between the two sites were not found to be significantly different. At Lee Bay, tidal variation was observed in behaviour, group size, and distance from shore. It is thought that Morte Point represents an important feeding area, while Lee Bay provides a corridor between more productive feeding sites. This study highlights the site-specific nature of diurnal and tidal trends as differences in habitat use were observed for two sites geographically close together.