Background & Objective: Yersinia spp is from Enterobacteriacea family. The members of this group are all gram negative bacilli or cocobacili and non-spore and negative heme-oxidase which casuses a vast spectrum of diseases in humans the commonest one of which is gastroenteritis. Other diseases and clinical syndromes are septicemia, mesenteric lymphadenitis, Appendicitis, pharyngitis and rarely Reiter's syndrome. Due to frequent reports of febrile gastro-enteritis from Health centers of Ardebil, the probability of gastroenteritis resulted from Yersinia in this cold province is predictable. Thus this study was done to determine the role of Yersinia spp in the diarrhea of children under 5 in the cold seasons of the year in Ardebil.

  Methods: Stool samples from 490 children under 5 with diarrhea in a 7-month period were collected, and transferred in carry-Blair to the laboratory. Stool samples were cultured on Mac-Conkey agar and Salmonella-Shigella agar and Yersinia selective agar, TCBS agar and selenate agar. After adequate incubation, the growing colonies were identified according to the standard biochemical methods.

  Results: of 490 samples, only 405 were suitable to be cultured of which 9.38% pathogenic bacteria were isolated. The frequency distribution of the separated bacteria were as follows Y.enterocolitica, 13 species vibrio, 11 species Enteropathogenic E.coli, 6 species Salmonella, 5 species Shigella, 2 species and plesiomonas, one species.

  Conclusion: The findings of this study are the indicator of the role of Yersinia spp in diarrhea in the cold seasons of the year in the cold and mountainous province of Ardebil. On the other hand, the frequency of isolated vibria was due to simultaneous cholera epidemy in this area.