Abstract: The differentiation of the ciliature and the base-associated fibers during asexual division of the fresh water ciliate, Onychodromus quadricornutus (Foissner et al. 1987) were observed by using protargol staining technique. During asexual division of O. quadriconutus, AZM and PM of the proster is formed at the site where the old oral structures is located and there is no detectable close relations between the new and old structures during the morphogenesis of left and right marginal cirri, the phenomena different from what found in some known oxytrichids. The cirri base-associated fibers of O. quadriconutus include anterior and posterior longitudinal fibers, transverse fibers and radiating fibers. The appearance of these fibers are similar to that observed in some known ciliates, except for that of the transverse fibers of left marginal cirri which differs from the known ciliates in location. During morphogenesis, the old ciliatures together with their base-associated fibers begin to degenerate and disappear in the primordial zone of new anterior and posterior FTV cirri. The base-associated transverse fibers of the old left and right marginal cirri are observed to be the first to disintegrate and degenerate. The disintegration of the base-associated transverse fibers in these parts of old structures shows a close relation with the differentiation of the primordial zone, suggesting an important role of this disintegration in supplying differentiation and development of the primordium with some substances. Other old ciliatures and the base-associated fibers can remain for a relative longer time in the cortex and finally begin to degenerate and disintegrate when their function declines gradually.