The rumen contents of 70 lactating Holstein cows fed five different rations were examined for ciliate protozoa. Species of eight genera of the family Ophryoscolecidae and two genera of the family Isotrichidae were observed, with Entodinium, Epidinium, Polyplastron, Ophryoscolex, and Isotricha predominating. Isotricha appeared frequently in the rumens of cows fed a ration containing cane molasses, whereas Polyplastron, Epidinium, and Ophryoscolex were frequently present in cows fed a ration containing hemicellulose extract. There were no differences in the frequency of appearance of most ciliate protozoa in the rumens of cows fed different quantities of calcium and phosphorus. Entodinium caudatum appeared frequently in the rumens of cows fed rations containing large quantities of grain starch.Polyplastron multivesiculatum or Ophryoscolex caudatus or both were not seen in cows containing Epidinium caudatum or Eudiplodinium maggii. However, O. caudatus was seen together with Epidinium tricaudatum. P. multivesiculatum was not seen together with Metadinium medium.