Both monolingual and bilingual children use learning constraints and heuristics to acquire new words from their environment. Overall, fast mapping abilities seem to be similar in both populations, but monolinguals rely more than bilinguals on the mutual exclusivity strategy. Our study probes the robustness of these results in a large group of children learning different language combinations, with a newly devised language-fair task that relies as little as possible on previous linguistic knowledge, in order to avoid disadvantaging bilingual children. We tested 138 3- to 5-year-old mono- and bilingual children in their dominant language (German, French, Italian, or Turkish) in a computerized task starting with a fast mapping phase, followed by a mutual exclusivity phase, using only invented nonobjects and nonwords. As hypothesized, monolingual and bilingual children showed similar results during the initial fast mapping stage, but monolinguals relied significantly more on the mutual exclusivity strategy than their bilingual peers. The language-fair design of our task supports the idea that differences in mutual exclusivity strategy use between mono- and bilingual children really stem from their specific linguistic background, and not from potential familiarity differences with respect to the test words. We discuss the implications of our results for bilingual language assessment in clinical or educational contexts. https://doi.org/10.23641/asha.20669214.