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Seven strains of bacteria were isolated from Pacific oysters, Crassostrea gigas, with a focal or systemic disease. The strains were aerobic, Gram-positive, acid-fast, produced a mycelium which fragmented into irregular rod-like elements, had a peptidoglycan containing meso-diaminopimelic acid, arabinose and galactose as major sugars, mycolic acids with 46-58 carbon atoms and G+C-rich DNA. All of these properties are consistent with the classification of the organisms in the genus Nocardia. A partial sequence of the 165 rRNA gene of isolate NB4H was determined following isolation and cloning of the PCR-amplified gene. The sequence was aligned with those of representative mycolic-acid-containing taxa and a phylogenetic tree was generated using the neighbour-joining method. It was evident from the phylogenetic tree that the three strains tested, RB1, OB3P and NB4H, were identical and belonged to the …