A survey of cnidarian material from Darwin Harbour, northern Australia, in the collection of the Museum and Art Gallery of the Northern Territory, yielded two species of commensal syllid polychaetes: Haplosyllis cf. bisetosa Hartmann- Schröder, 1960, an alcyonacean commensal previously known only from the Red Sea, plus a new species (and genus) of Syllinae, Alcyonosyllis phili. The new genus is characterised by having simple hooked chaetae with a subdistal boss and unarticulated dorsal cirri which are arranged in an alternating pattern of large, dorsally displaced cirri and smaller, lateral ones. Haplosyllis xeniaecola Hartmann-Schröder, 1993, which is also a commensal of alcyonaceans, is transferred to this new genus. Alcyonosyllis phili was found on nephtheid soft corals and gorgonians from the northwestern, northern and eastern coasts of Australia; it also occurs in southern New Guinea. Observations on the number of hosts harbouring A. phili and Haplosyllis cf. bisetosa and the number of worms per host are also provided.