Five species of the family Hapalidae were studied for the presence of marrow chimerism. In six of fourteen animals heterosexual cells were found in the bone marrow. Four species had a chromosome complement of 2 n = 46 while the pygmy marmoset (Cebuella pygmaed) is characterized by 2 n = 44. On morphologic grounds it may be presumed that the additional large metacentric element of the pygmy arose through centric fusion of small acrocentrics. Differences exist in the morphology of the karyotypes in these species, particularly with respect to the smaller elements and Y. Tissue cultures prepared from lung and kidney in three marrow chimerae contained no heterosexual cells. Two female animals with substantial marrow chimerism had normal external and internal genital tracts.