The paper aims to identify the specific features of mastering the grammatical category of gender by Russian-Yakut bilinguals of primary school age. The relevance of the study is determined by the general context of the development in the field of ontolinguistics as a fundamental science (the identification of the common mechanisms of mastering Russian as the first and a second language). In addition, the research relevance is conditioned by setting applied tasks in the field of the methodology of teaching Russian to bilinguals. The experiment performed with Russian-Yakut bilinguals of primary school age (the experimental groups), monolingual Russian preschool children, and adult bilinguals (the control groups) revealed that on the whole, the Russian-Yakut bilinguals of primary school age mastered the category of gender worse than the Russian-speaking preschoolers of three – four years old. The most difficult way to mark the category is syntactic: firstly, primary school students in Yakut (especially) and Russian classes in Yakutia make a large number of mistakes in the gender agreement between adjectives and nouns. Secondly, errors are detected in the agreement between past tense forms of verbs and nouns. Replacement of nouns with personal pronouns and agreement between nouns and possessive pronouns pose the greatest difficulties for primary school-leavers. Thus, teaching children speaking the Sakha language should be started with mastering the most ‘transparent’ ways of expressing the category of gender. These include lexical (comparison of the grammatical gender and the sex of people / animals) and morphological (marking gender in the ‘perceptually salient’ grammatical endings of nouns in the nominative case). It is also necessary to note the role of the Russian-language input (i. e. the speech of the people around, which children perceive and process): in case this input is substantial and of high quality, the category of gender is much more thoroughly mastered.