The presence of Yersinia ruckeri in rainbow trout hatchery farms was investigated by PCR Assay. Twenty-five rainbow trout, suspected of Enteric Redmouth Disease, were examined in this study. Yersinia ruckeri strains were identified from 8 out of 25 samples (32%). Ten of 25 (40%) rainbow trout samples showed positive reactions for Yersinia ruckeri in the PCR. Al l PCR positive samples have shown specific amplification bands of the 575-bp fragment for Yersinia ruckeri.