The relationship between the abundance of the toxic marine dinoflagellate Alexandrium spp. and cyst formation promoting bacteria (Alex-CFPB) was investigated in the water column of Hiroshima Bay (Japan) during 1997 - 98. All the sea water fractions collected from 5m depth where the density of Alexandrium cells was highest, and which also contained the bulk of planktonic bacteria, promoted cyst formation of A. catenella (Whedon and Kofoid) Balech. This promotion was not caused by nutrient limitation. The number of Alex-CFPB in the sea water samples, that was calculated by means of the most probable number (MPN) method, had a clear positive correlation with the abundance of Alexandrium spp. during blooms. 31 bacterial strains that have Alexandrium cyst formation promoting activity ("Alex-CFPB") were isolated. Population structure and genetic diversity of "Alex-CFPB" were analyzed by means of restriction fragment length polymorphism (RFLP) and partial sequences of the 16S ribosomal RNA genes. Fourteen ribotypes, A - N types, were observed among 31 strains of "Alex-CFPB" by means of RFLP with several restriction enzymes. Bacterial strains of ribotype A were dominant in the "Alex-CFPB" assemblages during the peak and termination periods of the Alexandrium bloom. The 16S rDNA based phylogenetic tree of the nine ribotypes among them showed that these fell within the alpha subdivision of the class proteobacteria. When encystment promotion activities of ribotype -A and -C isolates were analyzed in detail considering their effects on the algal growth, the encystment efficiencies (cyst yields/ maximum cell density) under the inoculation of each bacterium were almost 13 and 16 times as much as those under bacteria free conditions, respectively. These results suggest that Alex-CFPB play a significant role in the process of encystment and bloom dynamics of Alexandrium in the field.