Macaro has stated that the choice between a monolingual, immersive, target language-only pedagogy versus a non-immersive, multilingual pedagogy is 'probably the most fundamental question facing second language acquisition (SLA) researchers, language teachers, and policymakers'. Recognizing that prior empirical work on monolingual versus multilingual approaches has primarily been (1) descriptive, (2) in the context of English as a second or foreign language, and (3) very short term, often with one brief treatment, this intervention study examines the effectiveness of use of the L1/non-target language in the L2 classroom in a quasi-experimental, 10-week study examining French, a commonly learned foreign language, and Arabic, a less commonly learned foreign language, at beginning levels of proficiency in a community-based setting with 25 hours of instruction. Groups experiencing multilingual instruction outperformed those experiencing monolingual instruction in both languages with different instructors at almost all time periods and in almost all skill areas. Moderate to large effect sizes were found in inferential analyses of aggregated weekly progress quiz scores and scores in writing and vocabulary, and statistically significant differences between groups in Arabic were obtained in analyses of aggregated quiz scores overall and scores for vocabulary. These findings support theoretical position statements and a growing body of empirical research arguing for the potential benefits of inclusion of non-target languages in second language teaching and learning.