SUMMARY: In a suitable buffer at 39° the life of all three species of holotrich ciliates of the sheep's rumen can be extended for one or more days by addition of glucose, fructose, galactose, sucrose, cellobiose, raffinose, inulin, bacterial levan, salicin or melibiose (the least effective). Mannose, glucosamine or galactosamine are definitely toxic in that they greatly shorten protozoan life. This toxic effect is observed even in the presence of glucose. Isotricha prostoma and I.intestinalis, but not Dasytricha ruminantium, will ingest vegetable starch of small grain size, e.g. rice starch, thereby prolonging protozoan life. The nitrogenous requirements of these protozoa are best met by whole grass juice, which extends the life of the culture for several weeks; and even the ethanolic precipitate from boiled and cleared grass juice is a better nitrogenous supplement than cleared rumen liquor. The ash from this alcoholic precipitate will definitely extend protozoan life in absence of nitrogenous supplement. The effect is probably due to a ‘trace metal, which is not Zn, Fe, Sn, Sr, Mn, Cu or Ni, but may be Ti, Mo, Cr, Co or V. The holotrich ciliates here studied do not ingest lactobacilli, either when starved or when in an actively motile state after addition of glucose to the medium.