Urinary tract infections (UTIs) are one of the most common bacterial infections in humans, with ~80% of cases caused by uropathogenic Escherichia coli (UPEC)1. UPEC are increasingly associated with multidrug resistance, thus limiting treatment options2. Polymyxins are a group of cationic polypeptide antibiotics, with polymyxin B and colistin (polymyxin E) considered as last resort antibiotics for the treatment of infections caused by multidrug resistant UPEC2. Here we screened a saturated mini-Tn5 transposon mutant library of the well characterized multidrug resistant ST131 strain EC958 on media supplemented with polymyxins and employed transposon-directed insertion site sequencing (TraDIS) to elucidate molecular mechanisms of polymyxin resistance . The results showed that transposon insertion sites in mutants that grew in the presence of polymyxins were highly specific to one location at the 5’ end of the pmrB gene, which encodes a histidine kinase of the PmrAB two-component regulatory system involved in lipopolysaccharide modification. This insertion conferred resistance to polymyxins in EC958 by increased transcription of the pmrB gene. In parallel experiments, spontaneous polymyxin resistant mutants were selected by culturing EC958 in the presence of polymyxins. Whole genome sequencing of ten polymyxin-resistant mutants revealed they possessed mutations in either pmrA or pmrB, and in all cases this was associated with increased transcription of these genes. Overall, the data provide insight into the mechanism of polymyxin resistance in UPEC, and are consistent with the role of the pmrAB genes in polymyxin resistance in other Gram-negatives.