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The symbiotic assemblage associated with the tropical sea urchin, Salmacis bicolor was studied in the An Thoi archipelago, Vietnam. Five species of macrosymbionts were found on the surface of this echinoid host, including the polychaete, Ophiodromus sp. 3 (Hesionidae), gastropod, Vitreobalcis sp. (Eulimidae), shrimp, Allopontonia iaini (Palaemonidae), crab, Zebrida adamsii (Pilumnidae), and Ophiuroidea sp. This is the first record of A. iaini in association with S. bicolor in the coastal waters of Vietnam. The polychaete, Ophiodromus sp. 3 was located on the oral side of echinoids circled around the oral opening, while Vitreobalcis sp. was attached to the peristomial membrane. The shrimp, A. iaini and the crab, Z. adamsii were found among the spines; the latter was attached to the spines by chaelipeds. Of the 60 S. bicolor examined, 28 (46.6 %) were infested, with an average number of symbionts per host of 0.52. The prevalence increased with increasing sea urchin test size. We did not find symbionts on urchins whose test diameter was less than 35 mm. Almost all S. bicolor were inhabited by one symbiont, and in only three cases did we find two symbionts belonging to two different species on one host. Polychaete, Ophiodromus sp. 3 occurred most often, and only 1–6 specimens of the other species were found. The composition of the assemblage associated with S. bicolor was compared with that known for other assemblages associated with tropical sea urchins. The ecological characteristics of the association and the possible host-symbiont relationships are discussed.