Microcosm studies were performed to evaluate the effect of solid surfaces, bacterial adhesive ability, and inoculum size on colonization success and persistence of P. fluorescens or X. maltophilia, each with a Tn5 insertion that conferred resistance to kanamycin and streptomycin. In lake water enrichment microcosms, the presence of beads appeared to influence the abilities of P. fluorescens or X. maltophilia to colonize, as numbers of both organisms were greater in microcosms with beads (significant at same sampling dates at P < 0.01). However, solid surfaces did not appear to facilitate persistence of either organisms, as their numbers decreased with time in microcosms both with and without beads. The adhesiveness of the bacteria, as measured in an in vitro assay, did not relate to colonization success, as P. fluorescens and X. maltophilia colonized the microcosms to approximately the same level, but P. fluorescens was the more adhesive strain. In complex systems, colonization of surfaces appeared to result in higher numbers of organisms, but did not noticeably promote persistence. Adhesiveness of a particular organism is probably a relatively minor factor influencing its ability to colonize solid surfaces in complex natural environments.