The gene parC encodes the A subunit of topoisomerase IV of Escherichia coli. Mutations in the parC region analogous to those in the quinolone resistance-determining region of gyrA were investigated in 27 clinical isolates of E. coli for which ciprofloxacin MICs were 0.007 to 128 μ/ml. Of 15 isolates for which ciprofloxacin MICs were ≥1 μg/ml, 8 showed a change in the serine residue at position 80 (Ser-80), 4 showed a change in Glu-84, and 3 showed changes in both amino acids. No mutations were detected in 12 clinical isolates for which ciprofloxacin MICs were ≤0.25 μg/ml. These findings suggest that Parc from E. coli may be another target for quinolones and that mutations at residues Ser-80 and Glu-84 may contribute to decreased fluoroquinolone susceptibility.