The planktonic Rotifera are not all cosmopolitan in their geographical distribution. There is a latitudinal variation in the assemblage of planktonic species. Most of this variation can be attributed to species in nine genera, of which some (e.g. Brachionus) are more abundant in warm areas, and others are more abundant in cool areas (e.g. Notholca). The latitudinal change in the assemblage is gradual, but it can be measured by the Sorensen index of similarity.
In addition to the latitudinal variation there is evidence of a small group of species restricted to the Americas.
Detailed surveys extending over a year or more yield between 30 and 100 species for a given lake or area, except in the high Arctic where the number recorded for Spitzbergen falls to 16. Single samples yield many fewer species, normally ranging from 4 to 26.