A persistent and un usual bloom of Dinophysis acuta Ehrenberg in the Rías Bajas (Northwest Spain) from early July to mid-November 1989, was associated with a diarrhetic shellfish poisoning (DSP) outbreak that prevented mussel extraction in some areas from early August to mid-December. Integrated samples (0-5, 5-10 and 10-15 m) were taken once or twice a week at six stations. Maximum numbers of D.acuta (up to 2.3 X 10^4 cells 1^-1 in the integrated samples) and percentage of observed dividing cells (up to 31%), appeared in August,and formed a maximum in the thermoeline. Cell numbers were minimal when upwelling caused a breakdown of stratification. High numbers of D.acuta (up to 1.8 x 10^4 cells 1^-1y appeared again when persistentsouth winds in late October caused a reversal of the circulation. Thus, the two cell maxima occurred during two distinct hydrographic regimes, but in both cases at temperatures of 15-17°C. Numerous cells with one typical D.acuta valve, but the other valve reduced in size, were seen during the bloom, as well as Dinophysis dens Pavillard cells that might be a life cycle stage of D.acuta. Other observations include doublewalled, unwinged forms that are perhaps some kind of cyst, and a hypothetical 'fertiuzmg tube' emerging from the larger cell of a D.acuta/D.dens couplet. It is suggested that the maxima during stratification and during downwelling episodes could correspond to periods with a predominance of in situ growth and physical accumulation, respectively, and that the exceptional meteorological conditions during 1989 provided the optimum scenario for the unusual occurrence of D.acuta.

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