SUMMARY : Oligotrich protozoa from the sheep rumen, principally Entodinium caudatum, were maintained in culture in vitro and dividing every 2 days at a density of 15,000-30,000 protozoa/ml. for 18 months on a medium of rice starch, dried grass, 10 % (v/v) rumen fluid (fresh or autoclaved) and 50 pg. chloramphenicollml. The effect on these organisms of varying each of these factors in turn and the use of different growth conditions is described. The protozoa have also been grown from an inoculum of 100-900/ml. to a density of over 50,00O/ml. in 10 days. Many unsuccessful attempts to cultivate rumen protozoa in vitro have been made in the last hundred years; the earlier attempts were summarized by Becker, Schultz & Emmerson (1929). Margolin (1930) and Westphal (1934) achieved some success in the cultivation of oligotrich protozoa, but the first real advance was made by Hungate (1942,1943) who maintained some species of cattle oligotrich ciliates alive for over a year. Sugden (1953) was unable to maintain oligotrich protozoa from the sheep alive for more than 15 days, whereas Kandatsu & Takahashi (1955a, b, 1956) kept Entodinia spp. from goats alive for over 30 days with some multiplication. The purpose of the present paper is to describe experiments in which Entodinia spp., principally Entodiniurn caudatum from sheep rumen, have been maintained with frequent division for over 18 months; a preliminary communication has already appeared (Coleman, 1958). METNODS