BACKGROUND: Yersiniosis or enteric redmouth disease (ERM), caused by Yersinia ruckeri, is a serious bacterial disease in the farmed salmonids that causes economic problems in this industry. OBJECTIVES: This study was aimed to assess the experimental pathogenicity of Yersinia ruckeri in Rainbow trout (Oncorhynchus mykiss). METHODS: Two hundred Rainbow trout weighting 100-120 g, challenged with different strain of Yersinia ruckeri were obtained from affected trout farms using intra peritoneal injection route at a concentration of 108 cells/ml (0.1 mL per fish) to evaluate the virulence of these isolates. Each treatment group included 10 fish in two replicates and control fish received 0.1 mL sterile normal saline (0.9% NaCl). Following the intra peritoneal challenge, macroscopic and microscopic findings were determined. The most virulent strain was then used to determine the lethal concentration (LD50) using both intra peritoneal and bath method at dilutions of 103-1010 cells/mL. RESULTS: Macroscopically, anorexia, lethargy, circular swimming near the surface, blackening of skin, exophthalmia, hyperemia and hemorrhage in different parts of body, anal prolapse, enlarged liver and spleen were observed. Microscopically, hyperemia of hepatic sinusoids and vessels, necrosis and vacuolization of hepatocytes, increase in the abundance of macrophage centers in kidney, dilatation of Bowman’s space, degeneration and necrosis of kidney tubules, severe necrosis and detachment of intestinal villi, hyperplasia and clubbing of epithelial cells of secondary lamellae, spleen cell necrosis, goblet cell hyperplasia and thickening of epidermis layer in the tongue mucosa were observed.  The LD50 of intra peritoneal injection was calculated 1.2×106cells per fish 48 h post challenge. In bath route, LD50 was obtained 5×108 Cells/ml after 96 h. CONCLUSIONS: The results obtained from this study show virulence diversity of native strains.