Spontaneous and selected variants of Serratia marcescens with resistance against chloramphenicol, cotrimoxazole, nalidixic acid, and pipemidic acid, revealed moderately reduced susceptibility for enoxacin and norfloxacin, and slightly diminished susceptibility for ciprofloxacin. Therefore, urinary tract infections due to S. marcescens strains showing this nonspecific resistance mechanism still might be amenable to judicious chemotherapy with these three novel bacterial DNA gyrase inhibitors, particularly ciprofloxacin.