Dinophysis acuminata and D. acuta, the main agents of diarrhetic shellfish poisoning outbreaks in the Galician Rías Baixas, North-West Spain, appear segregated in time and/or space and exhibit considerable interannualvariability. To explore the specific requirements of the two species and the causes of their alternation in latesummer outbreaks, the meteorologically driven structure of the water column was examined in relation toDinophysis populations from monthly transects in Ría de Vigo and weekly sampling at a pilot station in Ría de Pontevedra during 2002 and 2003 — two years with contrasting meteorology. D. acuminata had a longergrowth season and reached higher concentrations in parallel with higher standing stocks of diatoms and dinoflagellates during 2002 compared with 2003. Its summer maxima were associated with thermohalinestratification and persistent northerly winds of daily mean velocities of 2.5–4.0m s–1. In contrast, during 2003, when the summer was very hot and the upwelling index was the lowest for the past 50 years, and the stratification was more intensively thermally driven, the latesummerpopulations of D. acuminata were replaced by those of D. acuta. The relationship between Dinophysis spp. and climatological conditions, as well as the more severe socio-economic impacts of 2003 relative to 2002,are discussed.