The purpose of this paper was to investigate the acquisition of morphology in the Croatian language. More precisely, how accurate does the primary school children (age 6-10) use different grammatical forms of low-frequency words. The target nouns were selected against different criteria: familiarity of a noun, frequency of a noun and its declension type. As familiar words were defined those words that pupils encounter in their obligatory home readings, while the unfamiliar words were randomly selected words that belong to the same declension type as the familiar words. The differencen between frequent and less frequent words was determined based on word-type frequency in Corpus HRWAC. The research participants were 113 pupils from elementary school Retkovec. The participants were students from six classes in total: two first grade, two second grade and two third grade classes. The method used was elicitation, and research instrument was designed for this particular research. Using visual prompts research participants were motivated to produce written and oral answers – morphologically correct form of a particular noun. From the results of the research it can be concluded that a certain number of errors appears in the answers of third grade students, but in a smaller number than by younger participants (the first grade and second grade). The research finding suggests that in the Croatian language morphological development does not stop at preschool age. The research results also proved that students are better acquainted with morphological forms of the words they know (words from obligatory readings), but not in the expected ratio. The students used correctly 66,75% of the familiar words and 60,18% of the less known words. Looking at the ratio of low-frequency and high-frequency words, the students used more correctly high-frequency words (58,85%), but with a small difference. Unlike them, they correctly wrote 53,75% of low-frequency words.