Yersinia ruckeri is the one of the most important causative agents of economic losses in fish farms in Iran. During April-August 2012، sampling was done from 285 rainbow trout weighing from 1-200 g up، from 19 coldwater farms that located in west-Azerbaijan province in order to isolate and identify the pathogenic bacterium Y. ruckeri, as causative agent of enteric redmouth disease. Samples from the fish's mouth, kidney tissue and the last third of intestine were cultured on yersiana selective agar media and then incubated at the 25°C for 48-72 hours. Using gram staining, biochemical tests and molecular assay (PCR) from 285 studied samples, 110 samples were infected to Y. ruckeri in the intestine (with an average of 59.38% contamination), 76 in mouth (with an average of 25.66% contamination)، and 25 in kidney tissue (with an average of 8.77% contamination). Moreover, Y. ruckeri were isolated from 7 farms out of 19 studied farms so the contamination rate about 36.84 percent was estimated. The results show that the average amount of contamination in west-Azerbaijan is higher than other provinces in the country، therefore; it can be the indicator of the deterioration and acuteness of the disease in west-Azerbaijan’s farms. Regarding different weights, the samples in finger size weighing 1-20 g were the most infected ones (52.63%) and the market size weighing up to 200 grams (24.56%) had minimal contamination with bacteria. This result revealed that fish with fingerling size are the most sensitive compare to higher weights. Also, significant correlation was observed between high rate of nitrite and disease occurrence (r=870 and p=0.03). It seems that amount of nitrite more than 0.1 ppm could be played important role as susceptibility factor in occurrence of ERM disease. So, doing proper health management could control the rate of disease outbreak in west-Azerbaijan’s farms.