A microbial community of a compost biofilter treating toluene vapors was investigated using serum-bottle assays and mineral-agar plates. Toluene was not consumed in the absence of oxygen. However, filter-bed extracts exposed to toluene vapor as the only carbon source produced distinct colony types (phenotypic groups) that were counted separately. Strains from each group were isolated and checked for toluene-degradation activity in serum bottles. Only 15% of colonies were true toluene degraders. This population was divided into 11 genotypic groups based on DNA fingerprints. Identification of a member of each group using 16S rRNA gene-sequence comparison showed that they belonged to seven genera: Acinctohncter, Azoarcus! Mycobacterium , Nevskia, Pseudomonas, Pseudonocardia and Rhodococcus. Together, members of the genera Pseudonocardia and Rhodococcus were 34 times more numerous than all the others. We hypothesized that these two organisms are K-strategists (adapted to a resource and crowded environment) and that the compost biofilter is a K-environment. This would explain why they are not outnumbered by faster growers like Pseudomonas or Acinetobacter species, which would be r-strategists (adapted to a resource-abundant and uncrowded environment).