This study examines whether Japanese native (L1) listeners can implicitly learn stress pattern regularities, not present in their L1, after a brief auditory exposure. In the exposure phase, the participants listened to and repeated words bearing stress patterned after Latin, but with a highly restricted consonant inventory. They performed a judgment task designed to test whether they had learned the relevant stress pattern regularities from the brief listening experience. We assessed participants' awareness of the inherent stress regularities by analyzing confidence in making decisions and by verbal report. Results suggest that although participants remained unaware of the underlying stress regularities they performed significantly above chance in endorsing correctly stressed novel items even when they claimed to be guessing. In addition, there was no difference in confidence between correct and incorrect judgments. These results suggest that brief exposure resulted in implicit knowledge of abstract stress assignment rules. However, participants rejected correctly stressed words that contained consonants that were not present in the test phase. On the basis of this finding, we speculate that in the acquisition of second language phonology, a violation at the segmental level may be deemed more costly than a violation in the prosodic domain.