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The article presents some morphological and semantic peculiarities of the category of number of Polish and Ukrainian nouns. In order to indicate the difficulties in teaching Ukrainian speaking people the category of number Polish plural nouns (mainly pluralia tantum, but also contexts of use of dual nouns, e.g. skrzypce, informacje, maliny, Czechy) were analyzed, together with their Ukrainian equivalents that are used in the singular (among them there are both singularia tantum nouns, and dual nouns, e.g. скрипка, інформація, малина, Чехія). Dictionaries of Polish and Ukrainian languages were used as the sources of information, as well as the resources of the National Corpus of Polish and Ukrainian Language Corpus of Texts.