It is common that biological wastewater-treatment processes are exposed to inputs of toxic compounds, such as phenolics. Due to their slow growth rate, nitrifying bacteria are most susceptible to inhibition that can lead to loss of nitrification capacity. Here, a microbial community containing nitrifying bacteria was acclimated to phenol, and it developed resistance to phenol inhibition and maintained nitrification activity. For the phenol-acclimated biomass, the NH₄⁺–N removal rates were almost unaffected when it was suddenly exposed to phenol. Heterotrophic synthesis and nitrification rates contributed 76% and 24% of the total NH₄⁺–N removal respectively during phenol removal, but the nitrification rate increased significantly once phenol was removed and mineralized. In contrast, the NH₄⁺–N removal rates decreased sharply for normal (unacclimated) nitrifying biomass when it was exposed to phenol. The phenol-acclimated biomass retained its resistance to phenol inhibition for at least two months after acclimation, and addition of the phenol-acclimated biomass to the normal biomass conferred resistance to phenol inhibition. Community analysis of the phenol-acclimated biomass showed an increase in families known to contain strains able to biodegrade phenolics. Taken together, the results indicate that the main impact of phenol acclimation was enrichment of phenol-biodegrading bacteria, which allowed rapid removal and mineralization of phenol and, consequently, alleviation of phenol's inhibition of nitrification.