The morphology and infraciliature of an endoparasitic ciliate, Boveria labialis, isolated from respiratory trees of the sea cucumber, Apostichopus japonicus, were investigated using living observation and silver impregnation methods. Based on the present and previous studies, an improved diagnosis is supplied: marine Boveria, size in vivo about 30–100×15–30 μm, body slender and flask-shaped, with a protruding lobe measuring 5–15 μm in length; one ovoid macronucleus and one micronucleus; single contractile vacuole positioned in posterior 1¼ of body length; 17–26 somatic kineties; paroral membrane and membranelle 2 forming a double anticlockwise spiral of nearly two turns.