Since first observed in 1956 the red tide blooms of Alexandrium minutum Halim remained a recurrent summer phenomenon in the type locality of the species, the eutrophic Eastern Harbour of Alexandria. The year 1994,however, has seen its last red tide, followed by the complete disappearence of the species. Daily observations in the summer of 1999 showed A. minutum to have been replaced by a community of potentially harmful species: Pseudonitzschia pungens, Prorocentrum minimum, P. triestinum and others. Alternate dominance of this community with diatoms, cyanophytes and euglenophytes is modulated by and significantly correlated to the wide salinity fluctuations, the wind velocity and the stability of the water column. The amplitude and duration of the bloom pulses is controlled by the grazing pressure of tintinnids, copepods and planktonic larvae.