The morphology and infraciliature of two novel marine ciliates, Frontonia mengi spec. nov. and Frontonia magna spec. nov., isolated from coastal waters in northern and southern China, respectively, were investigated using living observation and silver impregnation methods. Frontonia mengi spec. nov. is characterized by its slender body shape, with a length-to-width ratio of about 5 : 1, about 52 somatic kineties and the structure of its buccal apparatus. Frontonia magna spec. nov. can be recognized by the combination of huge body size, about 200 somatic kineties, five or six vestibular kineties and four-rowed peniculi 1-3. Phylogenetic trees based on small-subunit rRNA gene sequences were constructed by means of Bayesian inference and maximum-parsimony. Results showed that F. mengi and F. magna are sister to each other within the 'core' clade of Frontonia that also includes Frontonia lynni and Frontonia tchibisovae and that the genus Frontonia may be polyphyletic, because one species, Frontonia didieri, always groups with Paramecium and Apofrontonia.