1. The chromosome complement of Ellobius lutescens (2 N = 17 in both sexes) is the simplest known for higher mammals. 2. The author has tried to apply the criteria proposed in the Denver System to six mitoses (three of each sex) obtained from the bone marrow of animals which had previously been treated with Colcemid. 3. The results are no more exact than those previously obtained by the author (Matthey, 1957) on material which had not been treated with Colcemid. 4. The results seem to some extent applicable to the group C chromosomes (autosomes 6–12 and the X) of the human karyotype and justify the scepticism of Patau (1961) concerning the possibility of individually identifying all human chromosomes. 5. The comparison of the long, despiralized mitotic chromosomes (1957 material) with the contracted ones shows that the contraction induced by colchicine does not modify the relative proportions of the chromosomes nor the value of the centromeric index. These findings are in contradiction with the conclusions of Sasaki (1960