A two-wave longitudinal design was used to examine the extent to which aptitude predicts second language achievement. First-year Saudi university students (N = 56), who were involved in a seven-month intensive English language course as part of their degree, completed Aptitude and English proficiency tests at the start and the end of the treatment period. Univariate and multivariate analyses found moderate relationships between aptitude and second language achievement. Results are discussed for their implications for theory and interventions.