About 30% of 150 recent clinical isolates of Citrobacter freundii were resistant to greater than or equal to 3.13 micrograms of norfloxacin and ofloxacin per ml. Study of one quinolone-resistant strain for which the norfloxacin MIC was 100 micrograms/ml suggested that resistance was associated with both an altered A subunit of DNA gyrase and reduction in drug uptake accompanied by a decrease in an outer membrane protein.