Prior to their second birthday, children are sensitive to the semantic relatedness between spoken words. Yet, it remains unclear whether simultaneous second language acquisition affects this sensitivity. Here, we investigated the influence of early acquisition of two languages on the event-related potentials (ERPs) associated with lexical-semantic processing of spoken words in 18-month-old monolingual and bilingual children. Children were exposed to an auditory semantic priming task in French, while their ERPs were recorded. Word pairs were either semantically related (e.g., train-bike) or unrelated (e.g., chicken-bike), and they were presented at two stimulus onset asynchronies (SOA). The results revealed that only monolingual children exhibited a semantic priming effect at the short SOA while at the long SOA condition, both monolingual and bilingual children exhibited more pronounced ERPs in response to unrelated compared with related target words. This finding suggests that both language groups are sensitive to taxonomic relations between words but activation of semantic network might be less automatized or slower in bilingual children.