We used a combination of stable isotope probing (SIP), gas chromatography-mass spectrometry-based respiration, isolation/cultivation, and quantitative PCR procedures to discover the identity and in situ growth of soil microorganisms that metabolize benzoic acid. We added [(13)C]benzoic acid or [(12)C]benzoic acid (100 microg) once, four times, or five times at 2-day intervals to agricultural field plots. After monitoring (13)CO(2) evolution from the benzoic acid-dosed soil, field soils were harvested and used for nucleic acid extraction and for cultivation of benzoate-degrading bacteria. Exposure of soil to benzoate increased the number of culturable benzoate degraders compared to unamended soil, and exposure to benzoate shifted the dominant culturable benzoate degraders from Pseudomonas species to Burkholderia species. Isopycnic separation of heavy [(13)C]DNA from the unlabeled fraction allowed terminal restriction fragment length polymorphism (T-RFLP) analyses to confirm that distinct 16S rRNA genes were localized in the heavy fraction. Phylogenetic analysis of sequenced 16S rRNA genes revealed a predominance (15 of 58 clones) of Burkholderia species in the heavy fraction. Burkholderia sp. strain EBA09 shared 99.5% 16S rRNA sequence similarity with a group of clones representing the dominant RFLP pattern, and the T-RFLP fragment for strain EBA09 and a clone from that cluster matched the fragment enriched in the [(13)C]DNA fraction. Growth of the population represented by EBA09 during the field-dosing experiment was demonstrated by using most-probable-number-PCR and primers targeting EBA09 and the closely related species Burkholderia hospita. Thus, the target population identified by SIP not only actively metabolized benzoic acid but reproduced in the field upon the addition of the substrate.