SUMMARY.A 2‐year sampling programme of three benthic sites in a small eutrophic loch revealed large communities of ciliated protozoa consisting of a wide diversity of species (91 spp. over the 2 years). Species‐groups described with the aid of association analysis (normal and inverse) indicated the existence of a ‘ubiquitous’ species‐group consisting of at leastCyclidium glaucomaandAspidisca costata. Another species‐group containing large ciliates (Loxodes striatus, Frontonia leucas, Spirostomumspp. andParameciumspp.) was characteristic of the summer months at all sites. It is advocated that association analysis might increase the value of protozoa as pollution