Harbour porpoise (Phocoena phocoena) occur year round in the waters of southern British Columbia, but little is known about their seasonal abundance, habitat use and food habits. A systematic vessel-based line transect survey was undertaken to quantitatively assess seasonal trends in harbour porpoise abundance and distribution within the Canadian waters of Juan de Fuca and Haro Straits (08 September 2001 to 31 August 2002). These data were supplemented with opportunistic counts (1995-1996, 1998-2001) and stomach contents from post-mortem stranded porpoise (1998-2001). The study area encompassed 805.3 km2; with a total transect length of 1838.4 km. Data collection was restricted to Beaufort 0 and 1 sea conditions, and abundance was estimated using DISTANCE 3.5 software. Encounter rates observed from April to October were significantly higher than from the rest of the year. Seasonal abundance estimates (corrected for visual radial distance estimation) ranged from a high of 673 porpoise from April-October (CV=20.5%, 95% CI 450 – 1006) and declined to 208 porpoise from November-March (CV=37.5%, 95% CI 101 – 429). Harbour porpoise were not uniformly distributed within the study area. Localized areas of high counts may represent critical porpoise habitats. A bimodal distribution of stranding frequency corresponded to the period of increased abundance. Ten adult and five immature harbour porpoise stomachs were examined (8 males and 7 females). Fish bones and otoliths were identified to species. Each stomach contained only a single species of piscine prey (n=5). No cephalopod beaks or eye lenses were present. Specimens from south Vancouver Island contained sand lance (Ammodytes hexapterus), with the one from the southeast coast containing Pacific hake (Merluccius productus), and the one from the southwest coast containing Pacific herring (Clupea pallasi).