Müller, 1773 Paramecium is a well-known genus of ciliate protozoa, commonly studied as a representative of that group. The cells are slipper-shaped, and range from 50 to 300 μm in length, depending on species. Simple cilia covers the body, and there is a deep oral groove, containing inconspicuous compound oral cilia as found in other peniculids. Paramecium is widespread in freshwater environments, and is especially common in scums.

The division into species is still in flux - for instance, P. aurelia has recently been divided into 14 species - but the following are representative:

• P. aurelia Ehrenberg, 1838
• P. bursaria (Ehrenberg) Focker, 1836
• P. calkinsi Woodruff, 1921
• P. caudatum Ehrenberg, 1838
• P. duboscqui Chatton and Brachon, 1933
• P. jenningsi Diller & Earl, 1958
• P. multimicronucleatum Powers & Mitchell, 1910
• P. nephridiatum von Gelei, 1925
• P. polycaryum Woodruff, 1923
• P. putrinum Claparede & Lachmann, 1858
• P. trichium Stokes, 1885
• P. woodruffi Wenrich, 1928

A number of doubtful species have also been recorded.