Maristentor dinoferus n. gen, n. sp., was discovered on coral reefs on Guam in 1996 and has since been found frequently, at depths of 3-20 m. It forms black clusters, visible to the naked eye, especially on Padina spp. (Phaeophyta) and other light-colored backgrounds. When fully extended, this sessile ciliate is trumpet-shaped, up to 1 mm tall and 3 μm wide across the cap. The ciliate if host to 500-800 symbiotic algae. The anterior cap, or peristomial area, is divided into two conspicuous lobes by a deep ventral indentation. There is a single globular macronucleus, many micronuclei and, on average, 101 somatic ciliary rows and 397 adoral membranelles. M. dinoferus may be closely related to limnetic Stentor spp., but differs in two conspicuous features: (1) the cilia on the peristomial bottom are scattered (ordered rows in Sentor spp.) and (2) the paroral membrane is very short and opposite the buccal portion of the adoral zone of membranelles (in Stentor spp., it accompanies the entire membranellar zone). The cells appear dark due to stripes of cortical granules; the granules are more concentrated in a "black band" below the cap. The cortical pigment(s) is red fluorescent with a broad absorption peak in the yellow-green (ca. 550 nm) and red (600nm). Ultrastructural and molecular data demonstrate that the symbiont is a dinoflagellate of the genus Symbiodinium, the first unequivocal report of zooxanthellae in a ciliate. Phylogenetic analysis of a portion of the large subunit ribosomal RNA gene (28S rDNA) showed that the symbionts belong to Symbiodinium sp. clade C, a lineage that also inhabits many corals on Guam. The ciliate changes shape at night, and the symbionts, which are spread out in the cap during the day, are mostly with-drawn into the stalk at night; these changes were apparently not simply a response to darkness.