In inshore waters around Malta, Bonellia viridis inhabit burrows with multiple exits in calcareous rocks, and are most abundant in areas bordering Posidonia oceanica meadows. The associated epiflora and fauna are typical of the Mediterranean hardsubstrate infralittoral zone. The larger infaunal species associated with B. viridis include a poriferan, cliona sp. ; an unidentified nemertean worm; various polychaetes; the sipunculans Phascolosoma granulatum and Aspidosiphon muelleri; two species of molluscs --Lithophaga lithophaga and Lepidopleurus cajetanus; 8 species of decapod crustaceans; and the teleost Gobius geniporus. Four types of burrows are found in rocks containing B. viridis. The larger burrows (referred to herein as UBA burrows) contain 3 main species --the decapods Upogebia deltaura and Alpheus dentipes and B. viridis itself. Experimental evidence suggests that u. deltaura excavates the UBA burrows mechanically, although B. viridis may secondarily modify them by secretion of an acidic mucus and gentle mechanical action. There is a definite community of organisms living as commensals in the burrow of u. deltaura. The assemblage of organisms bears a striking parallelism to the assemblages inhabiting the burrows of species of Upogebia, Callianassa and the echiuran Urechis caupo of the Pacific coast of N. America. A food web for the UBA burrow community is suggested.