The growth and composition of microorganisms found in a municipal wastewater treatment plant were investigated in the presence of four pharmaceutically active compounds (PhACs) [ketoprofen, naproxen, carbamazepine, and clofibric acid] in batch reactors at varying organic loadings. Overall, the data suggest that microbial growth inhibition was correlated to organic loading rather than PhAC concentration. Significant inhibition ranging from 34 to 43% was observed under the lowest organic loading for all PhACs other than clofibric acid. No inhibition was observed at the highest organic loading. Higher microbial inhibition was not observed with increased PhAC concentration for a given organic loading. These results indicatethatthe presence of PhAC may affect microbial growth especially under lower organic loading conditions. Further validation is required with additional PhACs, organic substrates, and a wider loading range. In addition, significant microbial shifts were observed in the presence of ketoprofen and naproxen. These data suggest that, in addition to their effect on overall microbial growth, PhACs may affect the microbial ecology and additional research should be carried out to identify PhACs that have the potential of affecting ecologically important microorganisms in wastewater treatment processes and aquatic environments in general.