Trays with stones were implanted in a south Swedish stream. They rapidly became covered by large quantities of dcbris and detritus. The composition and diversity of the colonizing species collectives were examined after 2, 4, 8, 15 and 32 days. The changing substrate conditions were accompanied by a successive exchange of species demonstrating their capability of active habitat selection. The species diversity first rose but apparently approached an asymptotic level at the end of the experiment. Species richness was the major factor affecting the diversity, while the evenness component played a minor role, at least in more comprehensive samples.