Minimal inhibitory concentrations (MICs) of five antibiotics namely amikacin, gentamicin, tetracycline, ciprofloxacin, and nitrofurantoin for pork isolates of Yersinia enterocolitica increased two- to eightfold after bacteria were grown in the presence of 5 mM arsenite. For Y. enterocolitica isolates obtained from wastewater (sewage effluents), an unequivocal increase in MICs was seen with amikacin and gentamicin. No change was discernible in the outer-membrane proteins after isolates were grown in the presence of arsenite.