Ciprofloxacin, a fluoroquinolone-type broad spectrum antibiotic, was found to persist through wastewater treatment. This raises questions on how an antibiotic affects the native microbial community of wastewater treatment plants, and if this – in addition to hampering treatment – inadvertently contributes to antibiotic resistance; a continuing, severe challenge to medical procedures and treatment. We investigated isolates from the activated sludge communities in a Danish wastewater treatment plant to determine any ciprofloxacin-tolerant species, and discovered a number of strains able to grow when exposed to various concentrations of the drug.