Gyrodinium aureolum has been found responsible for a number of red tide outbreaks in northwest European shelf seas over the past decade, sometimes with adverse consequences on the ecosystem. The abundance of this species was followed up over several months in 1983 in the Bay of Douarnenez, from data obtained on a series of twelve cruises carried out from March 1983 to February 1984 and intended in the first place for the study of the influence of the Iroise coastal front on plankton production. High densities of Gyrodinium aureolum build up slowly: a three-month delay is observed from the appearance of the first cells in the Bay to the time when the abundance of the species reaches red tide concentrations. Different processes appear to have contributed to the generation of a red tide. Advection of low-salinity water into the Bay from the alongshore drift on the south coast of Brittany resulted in early stabilization of the water column and possibly introduced the species. Conditions favouring the dinoflagellate also persisted for several months, as thermal stratification (strong sunlight, no wind) replaced in June the spring haline stratification. The species could finally have benefited from intense regeneration which is shown to have taken place in the water mass below the thermocline.