A soil microcosm, containing a mixture of sand and a well-characterized phaeozem soil from loess, was developed for biodegradative applications. It was inoculated with soil-borne Alcaligenes xylosoxidans AB IV, degrading 2,2-dichloropropionate (DCPA), by a plasmidencoded haloalkanoic acid halidohydrolase. In long-term microcosm experiments, survival of Alcaligenes xylosoxidans ABIV in the presence and absence of selective pressure by the xenobiotic compound could be demonstrated as well as its capacity to maintain degradation of DCPA. At the same time its plasmid, pFL40, containing the degradative gene, dhlC, was horizontally transferred to Pseudomonas fluorescens in sterilized soil and also to different indigenous soil bacteria in nonsterilized soil.