Despite the growing interest in the phenomenon of learning without intention, the incidental learning of phonological features, especially prosodic features, has received relatively little attention. This paper reports an experiment on incidental learning of lexical stress rules, and investigates whether the resultant knowledge can be unconscious, abstract, and rule based. Participants were incidentally exposed to a lexical stress system where stress location of a word is mainly determined by the final phoneme, syllable type, and syllable weight. Learning was assessed by a pronunciation judgment task. Results indicate that participants were able to transfer their knowledge of stress patterns to novel words whose final phoneme was not previously encountered, suggesting that participants had acquired abstract and potentially rule-based knowledge. The combined use of subjective and objective measures of awareness in the present study provides a strong piece of evidence of the acquisition of implicit knowledge.