Trichodina zrechi n. sp. is a peritrichous ciliate which lives in the mid-gut of the echiuroid worm Urechis caupo Fisher and MacGinitie, 1928. Most of the worms used were taken from Drake's Bay (longitude 122? 57', latitude 38? 4'), California, but some were secured from Elkhorn Slough (longitude 121? 46', latitude 36? 49') and Morrow Bay (longitude 120? 52', latitude 35? 20'). The Trichodina are usually limited to the posterior third of the mid-gut of the host. This portion of the intestine differs slightly in color from the rest of the gut, and its contents are more viscous. It frequently contains numerous fecal pellets over which Trichodina actively move. The greatly distensible hind-gut, often filled with sea water, does not contain any protozoa, and the portion of the mid-gut anterior to the habitat of Trichodina contains a greenish or brown fluid which is apparently unsuited to the ciliate. It may be that the food material in the mid-gut has not yet reached a condition in which it is useful to the protozoa. It is also possible that this habitat is detrimental to the reproductive processes of the ciliate. If experimentally placed in this fluid, however, the protozoa live almost as long as when placed in fluid from their normal habitat. The method of infection of young worms by the commensal has not been determined. The usual method by means of a cyst suggests itself, but as yet encystment among Trichodina is unknown. No reproduction of the ciliate, taken from the host under laboratory conditions, could be detected. This may be correlated with the fact that in the laboratory apparently no ciliates emerge from the host. Discharges from the gut of Urechis were examined, and the water in which several hosts were living was repeatedly centrifuged and scanned without positive results. Since the protozoa live for but a short time in sea water, however, and may be quite sensitive to centrifuging, they may have disintegrated before their observation was possible. Small very young Urechis were found to be without Trichodina, which fact indicates that infection is associated with a certain size or stage in the life cycle of the host.