SUMMARYPopulations of over 400 Epidiniirm ecaudatum caudatumlml were maintained for over I 8 months in the presence of bacteria, in a potassium phosphate-rich medium under 95 % N,+ 5 % CO, with the daily addition of wholemeal flour and dried grass. Polyplastron multivesiculatum was also maintained at a population density of 45 to 70/ml under the same conditions, except that it was necessary to add each day a culture of another protozoon such as Epidinium ecaudatum caudatum, E. ecaudatum tricaudatum or Eudiplodinium maggii to give a population density of approximately roo/ml. The epidinia at least were engulfed by the polyplastron up to 10 being taken up each day; the minimum daily requirement for growth was one epidinium/polyplastron. Polyplastron multivesiculatum also grew in the absence of other protozoa on a sodium chloride-rich medium, and if these protozoa were inoculated into the potassium phosphate-rich medium in the presence of epidinia, there was a lag of 3 to 5 days before they began to engulf the epidinia. The effect of other variations in the culture conditions is also described. The present paper describes the cultivation of Epidinium ecaudatum caudatum and Polyplastron inultivesiculatum in the presence of bacteria in buffered salts medium with wholemeal flour and dried grass as the sources of carbohydrate. Under certain conditions P. multivesiculatum had an absolute requirement for the epidinia for growth and died rapidly without them.Preliminary communications gave the growth requirements of these ciliates immediately after their isolation from the rumen (Coleman, 1970(Coleman, , 1971).