Small experimental communities of bacteria and Protozoa were designed to test the widely held hypothesis that higher species diversity brings about greater stability. Three species of bacteria, three species of Paramecium and two species of protozoan predators, Didinium and Woodruffia, were used. The communities were maintained by regular additions of the appropriate combinations of species of bacteria. Stability was measured as persistence of all species and as a tendency to maintain evenness of the species abundance distribution. The measures were in essential agreement. Stability at the Paramecium trophic level was increased by increasing diversity at the bacterium level, but three species of Paramecium were less stable than two. An important finding was that one pair of Paramecium species consistently showed greater stability without the third species than with it. This finding indicates that there were significant second—order effects, with two species having an interaction that was detrimental to the third species. We conclude that much more experimental and observational work is necessary before the nature of any functional relationship between diversity and stability can be claimed with confidence.