The article presents results of a research experiment in which kindergarten children (24 people), primary schoolchildren (110 people), younger primary schoolchildren (I-III class) with retarded and underdeveloped speech abilities, especially articulation (30 people) and adult Polish language users as a control group (10 people) were included. The aim of the research was to answer the question, how the youngest Poles cope with Plural Genitive, which has numerous inflectional indicators, multi-functional and irregular endings. A great number of mistakes of all the experiment participants concerned difficulties in the use of such lexemes as:'pajac', 'koc', 'mysz', 'korytarz' (puppet, blanket, mouse, corridor). Forms considered as incorrect, according to dictionaries, were predominant, namely: 'pajacy', 'kocy', 'mysz', 'korytarzy'. As the research results show, qualifiers applied in dictionaries with forms of Plural Genitive of certain lexemes need some reflection on the part of linguists and, perhaps, even a change.