Chromosome preparations of the Olympia oyster Ostrea conchaphila Carpenter were studied using conventional Giemsa, silver staining, and C-banding techniques. The karyotype consists of six metacentric (1, 2, 4, 6, 8, and 10) and four submetacentric (3, 5, 7, 9) chromosome pairs. The silver-stained nucleolus organizer regions (Ag-NORs) were terminally located on the short arms of the submetacentric pair 5 (56% of cases) and on the long arms of submetacentric pair 7 (6% of cases). Constitutive heterochromatin was observed as telomeric C-bands on the short arm of the NOR-bearing chromosome pair 5 and as centromeric blocks of several chromosome pairs. Comparative analysis of patterns of karyotype, Ag-NORs, and C-bands of this species and of five other flat oysters, Ostrea angasi, O. chilensis, O. denselamellosa, O. edulis, and O. puelchana, for which data have been previously published, were performed, allowing the inference of cytotaxonomic relationships within Ostreinae.