Ten epidemiologically related Yersinia enterocolitica clinical isolates were studied. Six isolates were nalidixic acid resistant (MIC > 512 μg/ml), with mutations in the quinolone resistance-determining region (QRDR) of the gyrA gene, suggesting clonal dissemination of a nalidixic acid-susceptible Y. enterocolitica strain which has acquired different mutations generating resistance to nalidixic acid.