1. Four feeding trials were made to investigate relations between diet and protozoal population in the rumen.
2. When a ration containing no concentrate was used, the number of entodiniomorphs decreased rapidly. The number of entodiniomorphs increased with the amount of concentrate. Rice straw, which was used as a sole source of roughage, was not always necessary for protozoa to survive in the rumen.
3. The type of diet affected the holotrich population to a much smaller extent than the entodiniomorph population.