Abstract: Three marine pathogenetic ciliates have been morphologically studied:Nassula labiata Kahl,1933; Paranophrys carnivora Czapik & Wilbert, 1986 andCyclidium citrullus simile n.subspec.The first two organisms are new record in Chinawhich are either a red tidal form(Nassula labiata) or likely belonging to facultive parasitic forms (Paranophrys carnivora).Diagnostic features of Cyclidium citrullus simile n.subspec:small (12-16×7-11um) marine form with one macronucleus, contractive vacuole caudally located;10-11 bipolar somatic kineties with evenly distributed cilia; posterior part of SK(n-1), likethat in C.glaucoma, having close-set packed basal bodies.Ecological feature:saprobic marine water, salinity 31‰, temperature about 23℃.Type specimens as protargol impregnated slides are deposited in the Laboratory ofProtozoology, College of Fisheries, Ocean University of Qingdao, Qingdao 266003, P.R China.The new subspecies differs from the nominative Cyclidium citrullus citrullus Cohn,1865(=Cyclidium citrullus Cohn, 1865) n the different biotopes and smaller size(12-16vs.20-30um).We agree with some previous conclusions that the similar,well-known species, C.glaucoma Muller, 1786, should be considered as a strictfreshwater form, which can be distinguished from Cyclidium citrullus simile byevidently different body shape,habitats and larger size (25-30vs.12-16um).