Ibuprofen (IBP) is ranked at the 4th place among 57 pharmaceutical compounds according to the number of citations in prioritization documents. The response of microbial community of activated sludge to IBP was studied at the concentrations of 50-5000 mg/L. Batch incubation was performed in an OxiTop (R) device for 21 days. The reduction of biological oxygen demand depended on the IBP concentration and varied in the range from 321 to 107 mg O-2/L. Massive DNA sequencing analysis of the activated sludge revealed that Proteobacteria became more dominant when grown in the presence of IBP. Microbial diversity was reduced in the presence of 500-1000 mg/L IBP, but increased again in the presence of 5000 mg/L IBP, despite the domination of Enterobacteriales (48.1%) in this sample. Incubation of activated sludge in the presence of 1000 mg/L IBP led to an increased occurrence of ciprofloxacin-resistant bacteria. The use of Eosin Methylene Blue Agar for disc diffusion assay was shown to be more appropriate in order to reveal the changes in antibiotic resistance. The predominance of Enterobacteriales in the activated sludge is suggested as one of the possible explanations of the enhanced resistance to ciprofloxacin. (C) 2017 Elsevier B.V. All rights reserved.