The study investigated changes in the microbial population structure sustained at two different sludge ages of 10 d and 2 d under chronic impact of erythromycin. It intended to observe the experimental correlation between variable process kinetics and changes in the composition of the microbial community induced by erythromycin. Samples from two fill/draw reactors operated with continuous erythromycin dosing of 50 mg/L were collected for the analysis of microbial population structure using high-throughput sequencing of 16SrRNA gene. Significant changes were observed in the composition of microbial community during the exposure period. Richness analysis for slower growing culture indicated that most microbial fractions were inactivated and eliminated in favor of fewer more resistant species in different phyla. Sludge age appeared to control the impact of erythromycin on microbial composition. At a sludge age of 2 d, erythromycin appeared to generate richer community with faster growing and more compatible species. For slower growing culture, elimination of vulnerable species was supported by decrease in the number of shared level OTUs. For faster growing culture, shared species level OTUs also decreased significantly upon exposure to erythromycin, suggesting rapid washout and replacement by more resistant species. Resistance gene analysis yielded positive results for mph(A) gene indicating presence of erythromycin-resistant components in the microbial community. (C) 2014 Elsevier B.V. All rights reserved.