Using molecular genetic methods, we investigated the secondary contact zones of two pairs of species of ground squirrels of Mongolia. In common colonies of marmots M. sibirica and M. baibacina, we revealed a high frequency of occurrence of hybrid individuals that were viable and fertile. A hybridization between S. alaschanicus and S. pallidicauda was sporadic in nature. The hypothesis about an extensive hybrid zone in these species of ground squirrels was not confirmed. An occupation of suboptimal biotopes by individuals of different species was the main factor contributing to hybridization in both case. The complex social behavior in marmots affected on the localization and diffusion of the hybridization process.