The chromosome complement of the horse, the donkey and their two hybrids has been studied in somatic mitoses obtained by short-term cultures of peripheral blood leucocytes. The diploid number of chromosomes found was: horse sixty-four, donkey sixty-two, mule sixty-three, hinny sixty-three. In addition to the numerical differences between horse and donkey, there exist structural differences which are most noticeable in greater numbers of metacentric chromosomes in the donkey. These differences are reflected in the karyotypes of the hybrids, and partial assortment of parental chromosomes can be accomplished in mitoses of the mule and the hinny. It is possible that these physical differences play a role in synaptic failure during meiosis.