Variability in achievement across learners is a hallmark of second language (L2) learning, especially in academic-based learning. The Twins Early Development Study (TEDS), based on a large, population-representative sample in the United Kingdom, provides the first opportunity to examine individual differences in second language achievement in a longitudinal and genetically sensitive design, and to relate these differences to variability in first language achievement. Teacher National Curriculum ratings for Modern Foreign Language achievement at age 14 for 1632 twin pairs (611 monozygotic, 1021 dizygotic) constituted the core measure. Measures of socioeconomic status, early (ages 2-4) first language achievement (L1), and adolescent (age 12) L1 and reading significantly predicted L2 achievement. However, both individually and collectively they accounted for only a small proportion of the variance of L2 across the entire distribution. Twin analyses revealed that 42% of the variance of L2 was due to genetic variance, and about a third to shared environmental factors (shared within a family). Structural equation modeling confirmed that although there were genetic and environmental factors that influenced both L1 and L2 achievement (genetic correlations between L2 and other measures ranged from 0.33 to 0.51, and shared environment correlations ranged from 0.15 to 1.00), most of the genetic influence on L2 was independent of genetic influence on L1. Thus, the genetic factors that contribute to individual differences in L2 learning are largely distinct from those influencing L1. Additional analyses showed that the balance of genetic and environmental factors was very similar for the low achieving and high achieving students (lowest and highest 10%, respectively), and was similar to that for the full distribution. This pattern of results suggests that variability in L2 achievement is a continuous variable across the entire range, rather than reflecting qualitative or categorical differences at the extremes, and that it is largely the same genes which influence variability within the normal range and at the extremes. (Contains 5 tables and 1 note.)