ABSTRACTMorphology and morphogenesis ofOnychodromus grandis(Hypotrichida, Ciliata) were studied in vivo and in protargolimpregnated preparations. The species was isolated from halophyte communities in astatic shallow waters of the Hortobágy National Park, Hungary. The species has two well developed dorsal horns. According to some characteristics (number and situation of ventral cirri) our findings differ from Stein's description in 1859. Individuals of the population studied had 9 ventral cirri in 3 rows. Nine rows of dorsal kineties were found on the back. It was found that the oral primordium develops in the vicinity of the transverse cirri. InO. grandismarginal primordia are formed in the marginal rows with the participation of parental cirri. This is also true forLaurentiella acuminata.Dorsal fragmentation ofO. grandisdisplays similarity to that ofL. acuminata, considering especially the development of the numerous small dorso‐marginal kineties. There are certain similarities amongO. quadricornutus, O. indica, L. acuminata, Stylonychia voraxandO. grandisin the development of the ventral and dorsal pattern and in some morphogenetical events. The remarkable differences in the interphase cells can be related simply to the significantly different numbers of cortical units (number of cirri). and also to the various cell siz