Infraciliature and morphogenesis of six rumen ciliates, Ostracodinium mammosum, O. munham, O. dilobum, O. rugoloricatum, O. iwawoi, and O. tiete are described from pyridinated silver carbonate-impregnated specimens. These six Ostracodinium have a similar polybrachykinety arrangement, distinct from that of other ophryoscolecid ciliates and intermediate between those of O. gracile and O. damaliscus. Buccal infraciliature is composed of three polybrachykineties, a kinety loop, and paralabial kineties. Small dorso-adoral polybrachykinety, slender vestibular polybrachykinety, and kinety loop are characteristic. Dorsal infraciliature consists of the dorsal polybrachykinety that extends laterally along the dorsal side of the body. Morphogenesis is different from that of O. gracile, because the right end of ventral primordium extends dorsally to grow into a right primordium without separation.