The karyotype of Lemur fulvus albifrons, L. rubriventer and its hybrids with L. fulvus fulvus were compared with that of L. fulvus fulvus. The two subspecies of L. fulvus have the same karyotype. L. rubriventer differs from L. fulvus by five Robertsonian translocations and also by increased heterochromatin in the juxtacentromeric region of the X chromosome. The karyotypes in the present study compared with those of other lemurs allowed us to propose a general scheme representing the chromosomal evolution of the genus Lemur. In this genus Robertsonian translocations represent 29 of the 33 chromosomal rearrangements which have so far been identified. We cannot affirm that sterility of the hybrids is exclusively due to the accumulation of Robertsonian translocations.