An in situ combined monitoring system was used to examine the effect of a BTEX contamination on the diversity of a soil community in a BTEX-contaminated aquifer. Thus, mesocosm socks were filled with non-sterile and sterile uncontaminated aquifer material (from part A0) and positioned under the groundwater table in a monitoring well located in the contaminated area (part A1) of the site. Sterile and non-sterile A0 aquifer material inserted in A1 evolved towards the community structure characteristic for location A1. In all three cases, this structure was dominated by the same Pseudomonas species as determined by sequencing of 16S rRNA genes. The composition of the BTEX catabolic genotypes seemed to evolve similar in the sterile and non-sterile A0Z aquifer samples inserted at location A1, both developing mainly xylE-like genotypes next to the already present tmoA-like genotypes. This is an abstract of a paper presented at the 8th International In Situ and On-Site Bioremediation Symposium (Baltimore, MD 6/6-9/2005).