A bacterium was isolated from the pus of a patient with pelvic actinomycosis. The cells were strictly anaerobic, straight, non-sporulating, Gram-positive rods. It grows on sheep blood agar as non-haemolytic, pinpoint colonies after 24 hours of incubation at 37 degrees C in anaerobic environment. It is non-motile and does not produce catalase. 16S ribosomal RNA (rRNA) gene sequencing showed that there were 6.6% difference between the 16S rRNA gene sequence of the bacterium that of Actinomyces marimammalium (GenBank Accession no. AJ276405), a new species described in 2001, isolated from two seals and a porpoise. For these reasons a new species, Actinomyces hongkongensis sp. nov., is proposed, for which HKU8(T) is the type strain. Further studies should be performed to ascertain the potential of this bacterium to become an important cause of actinomycosis.