1. A major goal in ecology is to unravel how species assemblages emerge and how they are structured across the landscape. Host–parasite systems are particularly interesting in this context, as limited host dispersal may promote the differentiation of parasite communities. 2. We examined whether the patterns of species diversity in Cichlidogyrus, a genus of monogenean parasitic flatworms with a direct life cycle, are consistent with the hypothesis that parasite diversity is driven by host dispersal. This was carried out by comparing two sympatric cichlid hosts (Tropheus moorii and Simochromis diagramma) with contrasting dispersal abilities. Genetic connectivity among host populations along the Zambian shoreline of Lake Tanganyika was estimated using microsatellite genotyping. Cichlidogyrus parasites were isolated and identified morphologically to the species level. 3.Simochromis diagramma, a host with a high dispersal capacity, was infected by a low number of Cichlidogyrus species, and the parasite assemblages were similar among host populations. In contrast, T. moorii, a host with a low dispersal capacity, was infected by a large number of Cichlidogyrus species, and the parasite assemblages differed strongly among host populations. These outcomes were thus as expected from the hypothesis. 4. Because of the strong host specificity of these Cichlidogyrus species, a lack of connectivity among host populations might facilitate allopatric speciation of the parasite.

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