Following the new squash-technic ofMakino andNishimura, it is now possible to work more quickly and with more accuracy on the chromosomes of mammals. The author has established the cytological conditions in seven species of Muridae and elucidated some doubtful points belonging 15 other species.Microtus arvalis, M. orcadensis andM. incertus have the same formule and must be considered as very akin. The chromosomic set ofClethrionomys gapperi (North-America) doesn't differ from that ofC. glareolus (Europe). Three genera of Gerbillinae exhibit sex-chromosomes of the type found in palearctic Cricetinae. ByArvicola scherman andMicrotus nivalis a reinvestigation shows thatX andY are both metacentric. It seems preferable to putNeotoma floridana andPeromyscus leucopus, likely with the other genera of nearctic Cricetinac, in a special under-family, for they bear no cytological ressemblance with the true hamsters of the Old World.Dipodomys merriami, an Heteromyidae, has 70 and not 86 chromosomes (Cross, 1931). There is in none species coexistence of pre- and post-reduction, although pictures of chiasmas may be sometimes observed. The conception ofKoller andDarlington does not agree with the facts.