The purpose of the study was to evaluate the occurrence of genes directly connected with pathogenicity in Yersinia enterocolitica strains isolated from fattening pigs. Multiplex PCR, used for ystA, ystB, ystC, and ymoA gene detection, was optimised in order to determine the existence of the genes in one reaction. Material for the study consisted of 138 strains of Y. enterocolitica, which were preliminary examined by the bacteriological, sero- and biotyping methods, then bacterial DNA was isolated and multiplex PCR was performed. The presence of the products of length corresponding to the ystA and ymoA gene fragments was found in each of the examined strains. The ystB and ystC genes were not detected in any of the tested samples. The molecularly confirmed existence of Y. enterocolitica in fattening pigs indicates the carrier state and a possibility of shedding the microorganism into the environment, and shows that pigs are an important reservoir of the bacteria and a potential source of infection for humans.