Alexandrium minutum, a PSP producing species which recurrently blooms in the Bayona Bay, produced a toxic episode in September 1998, that was detected through mouse bioassays of mussels from the area. Bayona bay is a small inlet (ca. 4x3 Km) subsidiary of the Ría de Vigo. As soon as the toxicity in mussels was detected, a water sampling, that included six different sites in the bay, was started and maintained with a periodicity of two-three days until the nearly disappearance of the Alexandrium population, with the objective of determining the possible differences in toxin contents per cell and also those in toxin profiles, in and during the bloom. The obtained water samples were filtered through Whatman GF/C filters that were afterwards extracted. The PSP toxins were analysed by HPLC. The toxin profiles found were similar to those of the culture AL1V of the Instituto Español de Oceanografía, because they only contained GTX1 to 4 in significant amounts, although their relative proportions were slightly differents. Large differences were found both spatially and temporally. The total toxin concentration per cell varied more than one order of magnitude. The variation in toxin contents per cell, had a spatial component, changed by a factor of 5.2 and a temporal component, changed by a factor of 7.8. The possibility that those changes were produced by the environmental conditions was also studied.