Studies across a range of spatial and temporal scales are needed to discern multiple forces structuring communities. Subtidal prop roots of red mangroves host diverse assemblages of sessile marine epibionts that provide a model system for examining community development and maintenance at a variety of discrete spatial scales. During 1991—1992 we twice surveyed 11 sites at four cays in Belize, Central America, to quantify spatial variability and temporal change in distribution and abundance of root—fouling organisms at five sampling scales: (1) fronts and backs of roots (1—cm scale); (2) roots close to and extending away from peat bank (0.5—m scale); (3) along linear transects parallel to shore (1—50 m scale); (4) on leeward and windward shores of cays (0.5—km scale); and (5) among cays (1—10 km scale). Although epibiont community structure differed widely among sites, all cays surveyed had similar seasonal values of water salinity, pH, and temperature. Within cays, windward sites had higher dissolved oxygen levels and water flow rates than leeward sites. At still smaller scales, outer roots and fronts of roots received significantly more light and were subject to higher water flow rates than inner roots and backs of roots. Species richness, diversity, and mosaic diversity patterns indicated that epibiont assemblages were distributed non—randomly in space: leeward sites were more speciose than windward sites, and fronts of roots were more speciose than backs. Jaccard's index of similarity, cluster analysis, and Kendall's coefficient of concordance showed hierarchical patterns of decreasing similarity with increasing sampling distance. Significant spatial autocorrelation among Jaccard values occurred at 2—3 m intervals, possibly reflecting mean larval dispersal distances. Analysis of mosaic diversity among sites indicated the absence of a clear environmental gradient and supported the hypothesis that species distributions may reflect patterns of dispersal from initial source populations. While precise identity of species was unpredictable among roots, species groups based on taxonomy, morphology, and life history showed very consistent distributions among sites that may reflect variability in local root environments: algae were most prevalent in well—lit areas and on windward sites, while sponges and ascidians predominated in leeward areas. Relative importance and dominance of both individual species and species groups changed substantially between 1991 and 1992. Representatives of four species groups were transplanted across three spatial scales to assess whether post—settlement dynamics limit distributions of these taxa. All transplants survived well for the first 6 wk of the experiment. After 6 mo, all transplants exhibited similarly high levels of mortality regardless of treatment. Overall, the results indicate that larval supply may shape epibiont community composition on short time scales and small and very large spatial scales, while variation in physical factors may influence distributions over the long term and at intermediate spatial scales.