A chromosome study of the didelphiids C. lanatus and Ch. minimus is presented. The karyotype of C. lanatus is very similar to that of Marmosa robinsoni but differs, in so . far as the X chromosome is concerned, from that of C. derbianus. As an explanation for the karyologic difference between the two Caluromys species we postulate a peri- centric inversion of the X chromosome passing from the acrocentric form of C. derbianus to the submetacentric of C. lanatus. The chromosomes of Ch. minimus (2n = 22) are all acrocentric and have a strong similarity to those described in the genera Didelphis and Philander, but the sexual chromosomes are larger and have an XY form of sexual complement.