The present investigation was carried out to increase knowledge of the nitrogen fixing cyanobacterial communities of Thanjavur rice field, Tamilnadu, India. Totally ten different study sites were selected (Latituded 10°471N and longituded 79°101E) in Thanjavur district. At each site two water samples and one soil samples were taken samples were fixed in 4% formalin before taxonomic determination. A total of 18 filamentous heterocystous taxa were isolated of which 8 species have not previously been recorded in Thanjavur. During the study period the maximum species occurrence and abaundance of species belonging to Nostocaceae, Scytonemataceae, Rivulariaceae, Stigonemataceae show less occurrence in the all sites.