The effects of different phenol-feeding conditions on trichloroethylene (TCE) biodegradation and bacterial population structure in an aquifer soil community were studied. The soil sample, minerals, phenol, and TCE were mixed in glass bottles, which were then incubated under three different phenol-feeding conditions. First, phenol was supplied only once at 0.2 mM (condition 0.2P); second, it was added at 2.0 mM (condition 2.0): and third, it was periodically supplied ten times at 0.2 mM (condition 0.2PS). TCE concentrations remained stable under conditions UP and 2.0P. In contrast, TCE was completely degraded under condition UPS. TCE/phenol-degrading bacteria were enumerated indirectly and functionally by quantitative PCR. The low-K-s (half saturation constant) group of phenol-degrading bacteria, exhibiting high TCE-degrading activity, yielded a 50-fold higher population under condition 0.2PS than under condition 2.0P. The bacterial community structure under condition 0.2PS was studied by denaturing gradient gel electrophoresis targeting the genes encoding 16S rRNA and the largest subunit of multicomponent phenol hydroxylase. Sequence analysis of the major bands detected indicated the predominance of the low-K-s group of TCE/phenol-degrading bacteria belonging to beta-Proteobacteria. These results suggest that continuous supplementation with phenol at a low concentration increases the population of the low-K-s group of TCE/phenol-degrading bacteria.