The morphology, morphogenesis, molecular phylogeny, and the resting cyst morphology of the freshwater ciliate, Crassohymena primicirrata (Berger and Foissner, 1987) n. gen., n. comb., found in samples collected from South Korea, were investigated. The Korean two populations observed are identical to each other and are highly similar to the type population showing only few and minor differences in the size and arrangement of cortical granules and the length of dorsal kinety 4. The new genus Crassohymena n. gen. is established and characterized by a number of morphological and ontogenetic features: the thick undulating membranes comprising three or four rows of kinetosomes and in Cyrtohymena pattern
the anteriorly distinctly displaced transverse cirri
the posteriorly set-off cirrus V/3
the opisthe's anlagen IV-VI originate from cirri IV/2, V/4, and V/3, respectively
the proter's anlagen IV-VI originate from cirrus IV/3
and the enormous anlage I in each daughter cell. Phylogenetic analyses based on 18S rRNA gene sequences show that Crassohymena primicirrata n. gen., n. comb. is separated from all other morphologically most similar taxa and forms an adelphotaxon to the clade containing sequences of the families Neokeronopsidae and Spirofilidae, and the two oxytrichid genera Australocirrus and Notohymena.