S U M M A R YTwo sheep were reared together and then kept on a standard diet over a period of 9 months. The rumen of one sheep contained a mixed population of ciliate protozoal species ; the other was initially unfaunated, then faunated with Entodinium caudatum and then with Entodinium and Polyplastron multivesiculatum. For each sheep diurnal and day-to-day variations in total concentration, and concentration of particular types, of bacteria, in viable counts of bacteria on non-selective and differential media and in concentrations of the different protozoal species, were determined. Measurements were also made of in vitro rates of fermentation, rumen pH and ammonia and volatile fatty-acid concentrations. The presence of protozoa decreased rumen bacterial concentrations and increased ammonia and volatile fatty-acid concentrations. However, these changes were not always completely clear-cut since the protozoa changed the patterns of diurnal variation not only of bacterial numbers but of some metabolic activities and metabolites and also altered the balance of bacterial types. These results are discussed in the context of interactions between bacteria and protozoa and between protozoal species. The main effects of faunation of the rumen seem to be caused by non-selective ingestion of small bacteria by the protozoa.

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