This study investigated second language (L2) learners' perception of L2 sounds as an individual difference that predicted their improvement in pronunciation after receiving instruction. Learners were given explicit pronunciation instruction in a series of modules added to their Spanish as a foreign language curriculum and were then tested on their pronunciation accuracy. Their perception of the target sounds was measured with an AX discrimination task. Though the best predictor of pronunciation post-test score was pre-test score, perception made a unique and significant contribution. The other factors associated with better pronunciation of some L2 sounds were age, attitude, and time spent using Spanish outside the classroom. The results suggest that instructors should give adequate time for learners to hone their perception of target sounds at the outset of pronunciation instruction, because their initial ability to perceive the target sounds will, in part, determine how much they learn from such instruction. The results support models of L2 speech acquisition that claim that target-like perception is a precursor to target-like production, in this case in a formal learning context.