This study aims to isolate and investigate the natural algicidal bacteria. Sixty bacteria isolated from the coast of Chonburi Province and Bangpakong estuary with blooms of a dinoflagellate Noctiluca scintillans and diatom Skeletonema sp. These algal bloom associated bacteria were test for antibacterial and algicidal activity against standard bacterial strains and those algal bloom species. Two isolates IMS CH9 -1 and IMS W1-4  that were exposed to both algae at the initial density 3 levels at 10³, 10⁴, and 10⁵ showed growth inhibitory effect to wither and lyses the algal cells within 48 hours. Only isolate IMS CH9 -1 produced algicidal extracellular exudates. This strain was identified as Pseudoalteromonas sp. base on gene 16S sRNA with primer 16-23S_F (5’TTGTACACACCGCCCGTC) and 16-23S_R (5’CCTTTCCCT CACGGTACTG)

    Keywords: Algicidal bacteria, Noctiluca scintillans, Skeletonema, Pseudoalteromonas