The dispersion of harmful nonindigenous biological organisms that may be present in discharged ship ballast water is an issue of international interest. The present paper examines this issue as it applies to Vancouver Harbour and Juan de Fuca Strait, British Columbia, and the adjacent U.S. waters. The objective is to determine whether potential mechanisms exist to transport viable organisms that might be present in discharged ballast water to favourable reproductive habitats within British Columbian coastal waters. The study applied three-dimensional harmonic finite element models to generate representative tidal, atmospheric, and density-driven flow fields. Particle-tracking techniques were used to simulate representative trajectories of passive and active ballast water organisms discharged at existing deballasting sites. It was determined that the safest deballasting sites are off the west coast. Under normal conditions, organisms move southward (summer) or northward (winter) in the Shelf Break Current and only under strong eastward or northward winds are they transported to the Washington or Vancouver Island shorelines. Key words: ship ballast water, discharge, microorganism, trajectory, Vancouver Harbour, West Coast.