Thin sections of rumen ciliated protozoa of the subclass Spirotrichia were studied by electron microscopy to elucidate their ultrastructure. To prevent retraction of their adoral cilia, menthol crystals were used to relax the retrociliary region. These protozoa had a distinct ectoplasm and endoplasm with the macro-and micronuclei located in the ectoplasm. At the surface of the entodiniomorph body was a highly differentiated cortical zone of four layers. Ribosomes were abundant throughout the cytoplasm, suggesting a substantial potential for protein synthesis. These protozoa appeared to engulf bacteria into large vacuoles, and subsequently the bacteria were taken into the endoplasm in vesicles containing only one bacterium each. The bacteria were digested partially, and only in isolated cases were the bacterial cell walls still intact.