The living morphology and infraciliature of a marine ciliate, Certesia quadrinucleata, isolated from an abalone-farming pond near Qingdao, China, have been re-investigated, and some new information about this ‘well-known’ species documented: a caudally located contractile vacuole and the highly stable ciliary pattern on dorsal side. Based on previous and present studies, an improved diagnosis is supplied: size 50–90×35–70 μm in vivo; ovoid body shape with a sharp-angled projection on the right anterior edge; adoral zone with 22–27 membranelles and a prominent paroral membrane; 11–12 frontoventral and 4–5 transverse cirri; left marginal row with 5–7 cirri; five dorsal kineties; generally four macronuclear nodules; contractile vacuole positioned at posterior end; marine.