Yersinia (Y.) enterocolitica is considered as a frequent agent of human and animal diarrhea. One of the important virulence markers is the ystA gene encodes the production of enterotoxin YstA. However, not all the strains with ystA gene produce enterotoxin, which seems to be caused by the ymoA geneencoding the production of the YmoA protein inhibiting the expression of various genes. The purpose of our study was to evaluate the influence of the ymoA gene mutations on enterotoxin YstA production by Y. enterocolitica isolated from humans and pigs. All the studied strains obtained from pigs had the ystA gene which indicates that they belong to the group of strains commonly regarded as pathogenic, but the ability to produce YstA detected using infant mice bioassay was different (3 groups of strains: positive, doubtful, negative). The fragments of ystA gene were also detected in all Y. enterocolitica strainsisolated from humans and the ability to produce enterotoxin YstA were confirmed by the fact that all the strains were isolated from clinical cases of yersiniosis with diarrhea. Amplification of a fragment of the ymoA gene was also detected in all the studied strains, both humans and pigs, so we decided to sequencing ystA and ymoA genes to find possible mutation involving on enterotoxin YstA production. Mutations in ystA gene were not found, both in human and pig strains of Y. enterocolitica, but sequencing of ymoA gene fragments showed two mutations: transition A3387326G and insertion A in 3387368 position in some of examined strains. Detailed analysis of the strains belonged to each of 3 groups (positive, doubtful, negative) of Y. enterocolitica strains from pigs and similarly from humans showed no correlation between the occurrence of the ymoA gene mutations and enterotoxin YstA production by Y. enterocolitica strains.