The toxicity of trichloroethene (TCE) likely restricts microbial activity in close vicinity of a TCE dense non-aqueous phase liquid (DNAPL). This study examined the distribution of a dechlorinating community in relation to the distance from a TCE DNAPL using a diffusion-cell setup. Subcultures of the KB-1TM culture dechlorinating TCE to cis-dichloroethene and grown with either formate or lactate as electron donor were used to inoculate the diffusion-cells. 16S rRNA gene clone library analysis showed that both inocula consisted of dechlorinating bacteria similar to Geobacter lovleyi SZ and fermentative micro-organisms related to Clostridium and Clostridiales. qPCR and RFLP analysis of pore water and sand samples showed a stratified microbial community composition in the diffusion-cells. Geobacter dominated where TCE was dechlorinated, i.e. in the lower 3 cm of the 5.5 cm thick sand layer. Even at 0.5 cm distance from the DNAPL layer, Geobacter densities were two orders of magnitude higher than at inoculation, despite the expected TCE toxicity. In the upper 2.5 cm of the sand layer, where TCE was depleted, apparently fermenting populations prevailed, corresponding to Clostridium in some diffusion-cells. This analysis demonstrates that the microbial community composition in a source zone is related to the distance from the DNAPL.