Our aim was to investigate whether developing language system, as measured by a priming task for spoken words, is organized by semantic categories. Event-related potentials (ERPs) were recorded during a priming task for spoken words in 18- and 24-month-old monolingual French learning children. Spoken word pairs were either semantically related (e.g., train–bike) or unrelated (e.g., chicken–bike). The results showed that the N400-like priming effect occurred in 24-month-olds over the right parietal–occipital recording sites. In 18-month-olds the effect was observed similarly to 24-month-olds only in those children with higher word production ability. The results suggest that words are categorically organized in the mental lexicon of children at the age of 2 years and even earlier in children with a high vocabulary.