Ciliated protozoa comprise a diverse and quantitatively important component of salt marshes, especially in pools where microorganisms will tend to accumulate at the air-water and mud-water interfaces. The purpose of this study was to use artificial substrates as initially barren 'islands' and examine the colonization process in hopes of gaining a better understanding of the dynamics of the populations in these regions Mud from three different locations in the salt marsh near Leeville, La. was collected and placed in plastic pans in the laboratory. An overlying layer of seawater was added. Small nylon mesh grids (1.5 cm('2)) were placed parallel to the surface, either directly on the mud or just under the air-water interface. One grid (colonization grid) was collected and examined every two days for a period of three weeks. All ciliates on the grid were identified and counted. Additionally, a clean grid (invasion grid) was placed in the system each collection day and examined the next. These invasion grids gave an indication of the types and numbers of ciliates invading all grids over the interval between collections. Various physicochemical parameters were monitored Generally, community development follows the MacArthur-Wilson equilibrium theory. A dynamic equilibrium is attained with the species composition changing over time. There is a distinct succession of species from surface feeding forms to suspension feeding forms. The species composition and equilibrium number are also dependent on the origin of the mud. The invasion grids indicate that some species are excluded from the colonization grids, some are surviving on the colonization grids by a high invasion rate, still others are reproducing on the grids. Statistical analyses of association coefficients indicate the presence of species complexes The results of this study show that these artificial substrates do not just accumulate a random sampling of organisms, but that species interactions, environmental conditions and length of invasion all affect the developing 'island' community