Cyanobacteria were isolated from the North coastal areas of Vietnam, two strains of 43 samples 7ds and YBH1 suggested antibiotic properties. The experiments for the inhibition activity on some test microorganisms showed that the crude extracts of both two strains against test bacteria Bacillus subtilis ATCC 6633, Bacillus cereus A TCC21778, Escherichia coli PA2, Sarcina lutea M5, but have no effects on Candida albicans ATCC 10231 and Aspergillus niger 114. The biosynthesis of antibacterial substances reached maximum at 33 days of cultivation in the medium BG11 containing (g/l): (i) strain 7ds: NaCl 10.0, NaHCO3 3.1, K2HPO4 0.06, NaNO3 1.5, pH 7, under illumination condition of 10 hours: 14 hours light: dark cycle
(ii) strain YBH1: NaCl 10.0, NaHCO3 3.1, NaNO3 2.0, K2HPO4 0.06, pH. 10, light:dark cycle of 12 hours: 12 hours. Amplification and analysis of gene 16S rDNA of strains 7ds and YBHl then comparision with corresponding gene sequence of other cyanobacteria deposisted onto Gene Bank data resulted low homology of 81 percent and 86 percent, respectively. Based on their biological characteristics and according to Bergey (1989), strain 7ds has some morphological characteristics similar to genus Synehocystcis and strain YBHl similar to genus Gloeocaspa. These two cyanobacteria strains might be the new ones that do not belong to known cyanobacterial species. Further study should be carried out in order to classify and use them as producers of antimicrobial compounds for pharmaceutical application and others.