Many fresh-water protozoa can be found in litters and soils, but the ubiquitous species are those which are able to cope with fluctuating moisture conditions. Terrestrial protozoa are more characteristic of bryophyte-soil habitats than aquatic ecosystems. Nutritionally, two groups have evolved in response to the plant community: naked, predominantly bacterial feeders, whose abundance is determined by the decomposability of the litter in which they live; and the slow growing, humusassociated testacea, which are more abundant in the litters of slow decomposability. Ubiquitous species comprise about 90% of the protozoa in soils. More continuous moisture conditions enhance the appearance of additional species. Hence species diversity indicates higher moisture content of a soil. Protozoa may contribute to the functioning of the soil ecosystem by inducing fiocculation of bacterial populations and recycling of minerals through ingestion of bacteria and excretion of soluble products. The surface of vegetation appears to represent the most terrestrial habitat a protozoan can exploit, because in contrast to the litter-soil ecosystem, only one species, Colpoda cucullus, dominates the population.