Interference between non-native languages during trilingual language production

•In two experiments we examined language interference during trilingual production.•The non-native language (L3) caused more interference than the native language (L1).•Trilinguals applied more inhibition over their L1 than L3.•Interference between non-native languages might be related to reduced L3 inhibition. Most research on multilingual language control has focused on a bilingual’s first (L1) and second (L2) languages. Studies on third language (L3) acquisition suggest that, despite the L1 being more proficient, L3 learners experience more L2 than L1 interference. However, little is known about how a trilingual’s L2 and L3 interact after initial stages of language learning. In the current study (Experiment 1: 30 Spanish-Basque-English trilinguals; Experiment 2: 50 English-French-Spanish trilinguals), participants completed a speeded naming task to assess cross-language intrusions (e.g., using the Spanish “perro” instead of the French “chien”). Both experiments showed more L3 than L1 intrusions during L2 naming. Furthermore, using two different tasks, we assessed if this cross-language interference was related to language inhibition. Both experiments suggested that trilinguals inhibited their L1 more strongly than their L3. Together, this suggests that a trilingual’s non-native language might experience more interference from another non-native language than from their L1, possibly because trilinguals apply more inhibition over their L1.