Chromosomes of six cupped oyster species were studied using karyometric analysis alter conventional Giemsa staining, and silver staining, Karyotypes of Crassotrea angulata (nine metacentric and one submetacentric chromosome pairs), C. sikamea (nine metacentric and one submetacentric chromosome pairs), C. virginica (eight metacentric and two submetacentric chromosome pairs), C, ariakensis (eight metacentric and two submetacentric chromosome pairs), C, gasar (six metacentric and four submetacentric chromosome pairs), and Saccostrea commercialis (eight metacentric and two submetacentric chromosome pairs) are distinguishable by the number and position of the submetacentric chromosome pair and by the location of nucleolus organizer regions, Comparative karyological analysis of these six cupped oysters and of C. gigas was made using a Principal Component Analysis and a Hierarchical Clustering Analysis, Crassostrea gasar appears isolated trom the other oyster species, Then, two c1usters are separated. The first one groups C. gigas, C. angulata and C. sikamea, in which C. gigas is plesiomorphic, The second one consists of C, ariakensis, C. virginica and S, commercialis. Results are discussed with regards to oyster species relationships based on other genetic characters and to hybridization possibilities.