A unique project on the hybridization of domestic sheep with wild bighorn sheep of the Yakut subspecies (Ovis nivicola lydekkeri) was implemented under extreme conditions in Yakutia. Hybridization was performed to obtain new forms that combine productive indicators with specific adaptation qualities to harsh breeding conditions. Viable hybrids of both sexes were obtained via laparotomy during the artificial insemination of domestic sheep with frozenthawed epididymal male bighorn sheep. We compared the hybrids with domestic lambs. At birth, the hybrids had a live weight of 2.65±0.19 kg, which is 29 % less than that of domestic lambs. A difference in live weight was noted before reaching the age of 20–24 months; however, by 36 months, hybrid males outnumbered domestic rams of the same age. We found that in the early postnatal period, hybrids were satisfied with their mothers’ milk faster and more often, and their daytime sleep was short and shallow. The hybrids also showed a high degree of curiosity, jumping ability, caution, and clear imitation of the mothers’ behavior. In terms of growth and development, the hybrids were tall and narrow-bodied because of their high mobile activity, as well as differences in nutrition. The hybrid offspring differed in chromosome set from the parental forms, having a diploid set equal to 2n = 53, whereas the maternal form of the sheep had 2n = 54, and the paternal form had 2n = 52. The resulting hybrids are fertile, and differ from their peers in the dynamics of development, nutrition, and ethology. They have phenotypic features similar to those of the paternal form, and a new improved form of the genotype is the basis for selection in sheep breeding.