Although most children learn to communicate in their first language (L1), there is normal and expected variation in their rate of acquisition across all components of the language system. Until recently, most second language acquisition and second language (L2) researchers have assumed that individual differences (IDs) in L1 acquisition are small and irrelevant to IDs in L2 acquisition. However, growing evidence has found strong relationships between IDs in L1 attainment and ultimate L2 achievement. In this longitudinal study, US secondary students were administered measures of L1 written and oral achievement, L1 cognitive processing, and L2 aptitude, then followed over 3 years of Spanish instruction and administered standardized measures of L2 literacy and oral proficiency at the end of each year. Hierarchical regressions followed by regression commonality analysis showed that IDs in L1 achievement alone (reading, writing, vocabulary, and print exposure) accounted for substantial unique variance in L2 reading, writing, listening comprehension, and oral proficiency, while L2 aptitude accounted for additional unique variance at the end of each year. A new finding showed that variance accounted for by L1 skills increased from first to second to third year. Results lend additional support to the conclusion of strong L1-L2 relationships between IDs in L1 attainment and IDs in L2 achievement.