The aim of the present study is to evaluate  the effects of polyβhydroxybutyrate (PHB) on the growth performance, organ histomorphology , transcription of immune-related genes, and resistance of  cultured seabass (Dicentrarchus labrax) against Vibrio anguillarum infection. One hundred and twenty seabass were divided into four experimental groups in three replicates (n=3) fish  were fed on basal diets supplemented with 0.0, 5.0, 10.0, and 15.0 g PHB per kg diet for eight weeks. The final body weight, weight gain, and weight gain percent were significantly increased in all PHB-groups compared with the controls (P < 0.05). No significant differences were noticed in the specific growth rate , the feed conversion ratio and survival rate . There was a clear dose-dependent increase in the mRNA expression values of interleukin-10 and complement C5 genes in the hepatic tissues of treated fish (P < 0.05). After challenge with V. anguillarum, the relative percent of survival was significantly increased depending on the increase of dose of PHB in each group compared with the control group. Histopathological examination of gills showed ballooning of primary gill lamellae ,curling of secondary lamellae and hyperplasia of lamellar epithelium. Liver showed congestion of hepatic blood sinusoids, vaccuolation of hepatic cytoplasm with presence of bacteria in hepatic tissues. Posterior kidney showed swollen and vaccuolation of epithelium lining renal tubules. Intestine showed desquamation of villus mucosa , necrosis with presence of bacteria in submucosa.