The toxic dinoflagellate Alexandrium tamarense was studied along with surrounding oceanographic parameters from May 1998 to May 1999 in Scapa Flow, Orkney. Nutrient concentrations were higher in winter and showed a phase of depletion in late spring and summer. Dinoflagellates tended to be abundant in late spring and summer. Alexandrium tamarense was detected in the water column almost year round, with the exception of the winter months (September to December). The growth of the A. tamarense population in Scapa Flow was not reflected in a monospecific bloom but rather in moderate to small biomass levels and always in co-occurrence with other species of diatoms and dinoflagellates. The highest concentration recorded was 1600 cells l−1 but concentrations of 200–400 cells l−1 were more typical. The oceanographic parameters indicate that the occurrences of vegetative cells of A. tamarense seem to be explained in part by instability of the water column, low temperature and low nutrient concentrations.