Ubiquitous dispersal of free-living microbial species implies that each and every ecosystem supports a 'seedbank' of microbial species that are imported by random dispersal. However, many of the microbial species present in any particular ecosystem will probably never thrive there because the local environment is unsuitable for their population growth. To test this, we investigated the ciliated protozoa in a hypersaline lagoon in Almeria, Spain, using selective enrichment to reveal typical freshwater species, as the 'signature' of random dispersal. Twenty-four ciliate species, 14 of them not previously recorded from hypersaline waters, were identified in the undiluted waters of the lagoon. But when the salinity was gradually diluted, further species typical of fresh- and brackish waters emerged, indicating that they had persisted in a viable state at the previously high salinity. These additional species increased the recorded ciliate species total for the lagoon to 36. The species found in the lagoon appeared to be adapted to either high, or variable, or low salinity, implying that they may have originated in a variety of habitats that differed greatly with respect to salinity regime.