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¹ Karadeniz Technical University, Surmene Faculty of Marine Sciences, Trabzon, TÜRKİYE DOI : 10.4194/TRJFAS21726 Viewed : 1215 - Downloaded : 999 The aim of the study was to identify Pseudoalteromonas strain OS-9 and evaluate its anti-Vibrio potential. Based on the morphological, biochemical, and 16S rRNA sequence analysis, the strain was identified as Pseudoalteromonas haloplanktis. Experiments designed with Box Behnken showed that cell density and cell-free supernatant activity were simultaneously maximum at 30°C, pH 7, and 90 h. The coefficient values obtained from the response surface methodology regression equations determined that pH was more effective on responses than other physical variables. The cell-free supernatant of P. haloplanktis OS-9 extracted under optimized conditions was only effective on Gram-negative fish pathogenic bacteria and especially on Vibrio. Among Vibrio spp., the strongest inhibitory effect was observed against V. rotiferianus. At the maximum cell density (10.48 OD), the strain OS-9 showed the maximum inhibitory effect against V. rotiferianus (26.42 mm zone diameter). In co-culture assay, the growth of V. rotiferianus was inhibited after 96 h with an initial level of 1.0 × 10⁶ CFU ml-1 by the strain OS-9. These results indicated that P. haloplanktis strain OS-9 could be considered as a potential Vibrio bio-controlling agent. Keywords : Pseudoalteromonas Anti-Vibrio activity Response surface optimization