The features of long-term dynamics of black-capped marmot (Marmota camtschatica doppelmayeri) on the west macroslope of Barguzin Ridge in the "core" of the Barguzin Nature Reserve were examined. In 1997–2007, marmot’s populations were characterized with high population size and stable sex and age structure. Since 2010, significant reduction in family size, number of individuals in families and proportion of young individuals were observed. In some cases, we found complete disappearance of the colonies. The alleged reasons for reducing of black-capped marmot on the west macroslope of Barguzin Ridge were considered.