Recently, the cyanobacterium Anabaena has been observed to form bloom in reservoirs without artificial pollution sources such as the Urushizawa Dam. Morphological identification and sequencing of the 16S rDNA gene and rbcLX region of this microorganism were performed on 22 Anabaena strains isolated from the Urushizawa Dam and other lakes and reservoirs. Four species of Anabaena (A. circinalis, A. ucrainica, A. smithii and A. pseudocompacta), which are generally observed in eutrophic freshwater lakes and reservoirs in Japan, were isolated from the Urushizawa Dam. On the basis of the 16S rDNA and rbcLX gene sequences, the isolated Anabaena strains were classified into four and five clusters, respectively. Morphologically different species, namely, A. circinalis, A. ucrainica, A. crassa, A. mucosa, A. smithii and A. viguieri, were contained in the same cluster of the 16S rDNA tree. Most of the strains were contained in clusters that featured no capability for cyanotoxin production.
