An investigation into the distribution of Dinophysis spp. in coastal waters off the south coast of Ireland was carried out in July 2007. Dinophysis acuta was present as a sub surface layer containing up to 55,000 cells L-1. The population had a high percentage of viable cells (mean: 89%; median: 94%; n = 24) with a high specific division rate (~0.55 d-1). The layer, of approximately 5 m thickness, did not coincide with the fluorescence maximum and was present as a patch of horizontal dimension less than 10 km x 7 km. Both conventional and towed undulating CTD used in conjunction with high vertical resolution sampling methods showed the patch of Dinophysis to move with a similar speed and direction as the coastal flow, which ran parallel to the coast in the form of a coastal jet with speed of the order of 6.5–7 km day-1. The implications of the alongshore transport of populations of harmful species in coastal jets for monitoring programmes and predictive models are discussed.