The morphology, infraciliature and silverline system of an ectocommensal ciliate, Zoothamnium chlamydis sp. n., isolated from mantle cavity and on the shell surface of the cultured scallop, Chlamys farreri off the coast of Qingdao, China were studied from living and silver-impregnated specimens. The diagnosis for the new species is: marine Zoothamnium with alternatively branched stalk; zooids in vivo 50-90×25-60 µm, slender in shape with one layer of peristomial lip moderately everted, and bacteria covering the whole surface of the cell; one contractile vacuole apically positioned; macronucleus normally band-like, longitudinally oriented. Pellicle with conspicuous, widely spaced transverse striae. Number of silverlines from oral area to aboral ciliary wreath about 27-47; from aboral ciliary wreath to the scopula, 19-29. Zooids generally enlarged at both proximal and distal ends of branches.