Five ciliates, Chlamydonella prostomata nov. sp., Paractedoctema acruosa nov. gen., nov. sp., Urocyclon ovatum nov. gen., nov. sp., Porpostoma grassei (Corliss and Snyder 1986) and Cytharoides balechi Tuffrau 1974, collected from sea ice in the Weddell Sea, Antarctica were morphologically and taxonomically investigated. The new genus Paractedoctema is characterized as: Cyclidiidae with naked snout-like apical end and three well-developed membranelles which are multi-rowed in structure and closely apposed one to another; paroral membrane extending anteriorly to about mid-level of M1; one caudal cilium; and silverline system Cyclidium-like. Since Urocyclon Small and Lynn 1985 is a nomen nudum because no type species has been fixed, we re-establish the genus and give a revised definition: uronematids mostly with inverted pear-shaped body and subequatorially positioned cytostome; apical plate dominant; membranelle 1 highly reduced; and paroral membrane extending anteriorly to about mid-level of membrane 2. Based on this new definition, a new combination is suggested: Homalogastra cymruensis (Perez-Uz and Hope 1997) comb. nov. (formerly Urocyclon cymruensis Perez-Uz and Hope 1997). For the well-known genus Chlamydonella, an improved diagnosis has been given according to our observations and the data obtained: Lynchellidae without plasmatic protrusions on ventral side; several to many somatic kineties making no noticeable naked gap between left and right areas; perioral kineties continuous or slightly fragmented with leftmost rows parallel to each other, which are arched transversely; and cytopharyngeal rods (nematodesmata) toothed. Macronucleus usually dimorphic. Regarding the related genus Lynchella Kahl 1933, we suggest that the original diagnosis by Kahl should be maintained. Thus, the genus diagnosis is re-provided: Lynchellidae with plasmatic protrusions on ventral side; several to many somatic kineties making no noticeable naked gap between left and right areas; perioral kineties continuous or fragmented with some rows parallel to each other; cytopharyngeal rods toothed; macronucleus generally dimorphic. In the light of the redefinition, a new combination has been made: Chlamydonella nordica (Jankowski 1968) comb. nov. (formerly Lynchella nordica Jankowski 1968).