Karyotypes of Dicrostonyx torquatus torquatus and D. vinogradovi that are very similar phenotypically differ at least by 13-14 chromosomal rearrangements including centric and tandem fusions, fissions and paracentric inversion. In their male hybrid F1 spermatogenesis is arrested on early pre-meiotic stages, the female hybrids are semisterile. Histological investigation of ovaries has demonstrated reduced number of follicles, higher percentage of atretic follicles and an acceleration of folliculogenesis in hybrids as compared with the parental forms. As a result of the acceleration, the number of mature follicles and ovulations is not significantly diminished in the hybrid females, and their semisterility is caused, to a great extent, by high prenatal loss which most likely arises from the chromosomal imbalance in F2 hybrids. Thus, the mechanism suggested by the concept of chromosomal speciation is active, but not alone in forming reproductive isolation between twin species of the varying lemming.