Immigrants’ first language proficiency (L1) is increasingly seen as a resource. However, empirical findings on the role of L1 proficiency in immigrant students’ school success are ambiguous, and more reliable evidence on this issue is needed. We therefore developed L1 proficiency tests for second graders within the National Educational Panel Study (NEPS), a large-scale study conducted in Germany. The tests assess listening comprehension in Russian and Turkish, the L1s of Germany’s two largest immigrant groups. The present paper describes the NEPS-L1 tests and investigates their validity in two studies (Study 1: N1Russian = 105/ N1Turkish = 145, Study 2: N2Russian = 217/ N2Turkish = 152). Findings on the factorial validity suggest that the tests are global measures of L1 listening comprehension proficiency. Correlations with convergent (e.g., L1 vocabulary) and discriminant (e.g., nonverbal reasoning ability) validation criteria yielded the expected patterns. DIF analyses further revealed that the majority of test items were of similar difficulty for both language groups. We thus conclude that the NEPS-L1 tests are valid measures of second graders’ listening comprehension in Russian and Turkish and can provide important data for research on L1 proficiency.