This study investigated the relationship of first language (L1) skills in elementary school and second language (L2) learning in high school. Students classified as high-, average-, and low-proficiency L2 learners were compared on L1 achievement measures of reading, spelling, vocabulary, phonological awareness, and listening comprehension administered in the first, second, third, and fifth grades. An L2 aptitude measure was administered in ninth grade and L2 word decoding and L2 spelling measures were administered at the end of the first- and second-year L2 courses. Outcome measures were oral and written L2 proficiency measures in Spanish, French, and German administered at the end of 2 years of L2 study. Results showed overall differences among the three proficiency groups on the L1 achievement measures from second through fifth grades, the L2 aptitude measure, and the L2 word decoding and spelling measures. High-proficiency L2 learners exhibited stronger L1 skills and L2 aptitude than the average and low-proficiency L2 learners. Findings showed that L1 skill differences emerged early in elementary school and are related to L2 proficiency and achievement several years later in high school. The findings provide support for long-term crosslinguistic transfer of L1 to L2 skills.