Oxytricha fallax and Stylonychia pustulata possess 6 rows of dorsal bristle units. Each dorsal bristle unit consists of a pair of kinetosomes; the anterior kinetosome has a cilium and the posterior kinetosome a ciliary study. The kinetosome pair, located at the bottom of a cortical pit surrounding the cilium and ciliary stub, is surrounded by an asymmetrical fibrillar mass. Future rows 1-4 are formed from 2 sets of primordia originating within mature dorsal rows 1-3. Rows 5 and 6 originate from the anterior regions of both right marginal cirral primorida. Old dorsal bristle units utilized in formation of primordia are presumably maintained in the new rows of the proter and opisthe; those outside the primordia resorbed. The morphogenetic pattern of the Oxytrichidae is similar to those of the Urostylidae and Holostichidae, but quite different from that of the Euplotidae.