The karyotypes of Rangifer t. tarandus L . and Rangifer t. fennicus Lonnb. have been established. The chromosomes have been stained by several methodes (G-, C-, Qand R-banding, NOR-staining). The chromosome number is 70, N F is 74. The autosomes consist of 33 acrocentric pairs and one submetacentric pair (no. 34). The sex chromosomes, X and Y, are of giant size. The karyotype is identical in both subspecies (Gripenberg et al. 1984). The largest autosomes (nos. 1 and 2) have tiny satellites; satellite associations are frequently observed in the metaphases. The idiogram of the reindeer is compared to the idiogram of the moose (Alces alces) (Gripenberg et al., in press). Several similarities are found. The chromosome number in the moose is 68 as a consequense of a fusion between two acrocentric chromosomes forming a large new metacentric chromosome (no. 32). N F is 74 as in the reindeer. Many of the reindeer autosomes have G-band patterns identical with the patterns of corresponding moose chromosomes. The most obvious difference between reindeer and moose concernes the amount and distribution of heterochromatin (C-bands). The autosomes of the reindeer have small C-bands; the C-bands of the moose autosomes are large, comprising approximately 40% of the total amount of the chromatin. Heteromorphic C-bands are frequently seen in the moose karyotype. The two largest autosomes of the moose are satellited as the corresponding chromosomes of the reindeer. The large arms of X and Y chromosomes of the reindeer have huge heterochromatic blocs; in moose the sex chromosomes have insignificant C-bands. The X chromosome of the moose has during evolution developed through an inversion of an originally acrocentric X chromosome. This ancient X chromosome is still present in those cervids with the original karyotype (Neitzel 1982). This X chromoxome differs according to its banding pattern decisively from the common mammalian X chromosome. Polymorphic Y chromosomes have furthermore been observed in the moose. The submetacentric autosomes (no. 34 in the reindeer and no. 33 in the moose) are identical. They lack C-bands. This chromosome originates from a large acrocentric autosome, which has undergone a pericentric inversion. Satellite D N A has been isolated from the reindeer and moose genomes. In situ hybridisation of the cloned D N A fraction is in progress.