Applied linguistic work claims that multilinguals’ non-native languages interfere with one another based on similarities in cognitive factors like proficiency or age of acquisition. Two experiments explored how trilinguals regulate control of native- and non-native-language words. Experiment 1 tested 46 Dutch–English–French trilinguals in a monitoring task. Participants decided if phonemes were present in the target language name of a picture, phonemes of non-target language translations resulted in longer response times and more false alarms compared to phonemes not present in any translation (Colomé, 2001). The second language (English) interfered more than the first (Dutch) when trilinguals monitored in their third language (French). In Experiment 2, 95 bilinguals learned an artificial language to explore the possibility that the language from which a bilingual learns a third language provides practice managing known-language interference. Language of instruction modulated results, suggesting that learning conditions may reduce interference effects previously attributed to cognitive factors.