Abstract: The sequence of rDNA Intergenic Spacer Region (ISR) was determined for the species Oscillatoria sp.(Cyanobacteria). The spacer sequences was 427bp in lengh exclusive of 16S and 23S rDNA coding regions, containing a Ile tRNA gene. Based on the analyses of the obtained sequences and the sequences from other three representative species of Oscillatoriaceae, and two species of Microcystis , the evolutionary affiliation among the genera of Oscillatoriaceae was discussed. In addition, the results showed that rDNA Intergenic Spacer Region can be served as a criteria for the identification or the classification of Cyanobacteria within the genus or intraspecies, which may be a useful genetic marker at the species or popu-lation level and may be one of the most suitable regions in the rRNA gene cistron for preparing DNA probes specific to Cyanobacteria species because of the high divergence of this region.

Key words: Oscillatoriaceae