Triplicate, 30-litre plastic microcosms containing pasteurized pond water and polyurethane foam substrate sampling devices were placed at 10, 32, and 100 m from an isolated, ephemeral pond near the Virginia Polytechnic Institute & State University campus, Blacksburg, Virginia. A single substrate was collected from each container on days 3, 9, 15, 22, 29, and 36 and sampled for the number and kinds of protozoan species present. The process of species accrual in the pond and the containers was adequately explained by the MacArthur-Wilson equilibrium model. Dummy variable analysis revealed no difference in the rate of colonization among the three distances, but the estimated number of species at equilibrium decreased significantly at 100 m from the pond. The total number of species identified during the experiment decreased with increasing distance from the pond, as did similarity with the pond fauna, also suggesting a distance effect with respect to the pond. Extinction and immigration curves fitted as a function of time suggested that the probability of a species successfully colonizing an island increased as early stages of community development proceeded. Data presented for microbial species accrual in isolated systems appear to be best explained by an interactive model of colonization; the successful colonization of pond species appears to be dependent on the initial arrival of pioneer species, most of which are present in the soil and atmosphere.