Epibiont ciliates on the exoskeleton of the decapod crustacean Liocarcinus depurator, a portunid crab collected off the Mediterranean coast of Spain, included 11 species, belonging to three subclasses. The chonotrich Chilodochona quennerstedti, the most abundant species, colonized specific sites on the crab, the mouthparts. The peritrichs, identified as Zoothamnium sp. and Cothurnia sp., and the suctorians, identified as Acineta tuberosa, Acineta papillifera, Ephelota gemmipara, Actinocyathula homari, Paracineta limbata, Conchacineta constricta, Corynophrya anisostyla, and Acinetides symbiotica, were generally less site-specific. Feeding activities, burying behavior, and molt state of the host crab, and competition between ciliate species, are mentioned as the most important factors in the occurrence of ciliates on the crustacean body. Additional key words: epibiosis, Chonotrichia, Peritrichia, Suctoria, burying crab Decapod crustaceans are frequently hosts, or basibionts, of diverse sessile invertebrate metazoans (hydrozoans, bryozoans, cirripeds, polychaetes, etc.) although, to some extent, the molt of crustaceans, especially young, fast-growing individuals, keeps the exoskeleton free of epibionts (Ross 1983). Protozoans constitute another group of epibionts, probably less well known (see annotated list of parasite and ectocommensal protists of decapod crustaceans, Sprague & Couch 1971). Several subsequent studies on parasitic protozoans and diseases caused in some peneid decapods of economic importance (Overstreet 1987) have noted mainly the pathological effects of the presence of epibionts. Also, many ciliate species-apostomatids, peritrichs, suctorians, heterotrichs, and chonotrichs-have been reported as present on crustaceans (Collin 1912; Fenchel 1965; Corliss 1979; Batisse 1986, 1992; Clamp 1991). However, most of these studies have focused on describing the morphological features of the protozoans; in few cases have the ecology and the relationships with the host been considered. Many sessile marine organisms depend on the physical characteristics of the substrate to which they are attached (Gili et al. 1993); hence, the structure and dynamics of the substrate is reflected in the pattern of colonization by epibiont species. In the same way, epibiosis can contribute to the description and explanation of important aspects of the biology and ecology of basibiont crustacean species (Bottom & Ropes 1988; Abello et al. 1990; Abello & Macpherson 1992; Gili et al. 1993). Molt cycles and reproductive behavior are some of features studied using macroepibionts and their distribution patterns. However, this type of study does not take into account the epibiont protozoan communities. Liocarcinus depurator LINNAEUS 1758 is a portunid crab living in muddy beds of the continental shelf, at depths between 1 and 100 m (Abello 1986). During the period between the molts, these crabs are an available substrate for epibionts. We have described the ciliate groups most commonly found on the exoskeleton of L. depurator and their distribution on the crab's body. The settlement and development of these protozoan communities is determined not only by environmental conditions, but also by the biology and behavior of the crustacean.