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Cells in short-term culture from 12 species of pinnipeds were submitted to chromosome analysis, including, in 11 of them, chromosome measurements and establishment of idiograms. Analysis of variance was carried out on the measurements of 9 phocids. Comparisons between the different karyotypes were made on the basis of conventional chromosome methods as well as autoradiography, G- and C-band techniques. The Pinnipedia are characterized by a pronounced karyotype uniformity. Within the Otarioidea, all otariids so far studied have 2n = 36 and closely agreeing karyotypes, the walrus has 2n = 32. In the Phocidae 2 chromosome numbers occur, 2n = 34 and 2n = 32. The 2n = 32 karyotype has evolved from the primordial 2n = 34 karyotype through fusion of two pairs. Within each of these two groups all karyotypes are virtually identical. The morphological and banding homologies between the otariid and 34 chromosome phocid karyotypes confirm the theory of monophyletic origin of the Pinnipedia.