We present one of the first behavior-genetic studies of individual differences in school students' levels of achievement in instructed second language acquisition (ISLA). We assessed these language abilities in Australian twin pairs (maximum N pairs = 251) by means of teacher ratings, class rankings, and self-ratings of proficiency, and used the classic twin design to estimate the relative influences of genes, shared (family/school) environment, and unique environment. Achievement in ISLA was more influenced by additive genetic effects (72%, 68%, and 38% for teacher ratings, class rankings, and twin self-ratings, respectively) than by shared environment effects, which were generally not substantial (20%, 07%, and 13%). Genetic effects distinct to speaking and listening, on the one hand, and reading and writing, on the other, were evident for the twin self-ratings. We discuss the limitations and implications of these findings and point to research questions that could profitably be addressed in future studies.