Do the cognitive benefits of "interleaving"--the method of alternating between two or more skills or concepts during training--extend to foreign language learning? In four experiments, we investigated the efficacy of interleaved versus conventional blocked practice for teaching adult learners to conjugate Spanish verbs in the "preterite" and "imperfect" past tenses. In the first two experiments, training occurred during a single session and interleaving between tenses began during the presentation of introductory content (Experiment 1) or during randomly ordered verb conjugation practice trials at the end of the training session (Experiment 2). This yielded, respectively, numerically higher performance in the blocked group and equivalent performance in the interleaved and blocked groups on a 2-day delayed test. In Experiments 3 and 4, the amount of training was increased across 2 weekly sessions in which the blocked group trained on 1 tense per session and the interleaved group trained on both tenses per session, with random interleaving occurring during verb conjugation practice trials. Interleaving yielded substantially better performance on a 1-week delayed test. Thus, although interleaving did not confer an advantage over blocking under 2 different single-session training schedules, it improved learning when used to practice conjugating verbs across multiple training sessions. These results constitute the first demonstration of an interleaving effect for foreign language learning.