<p>The current thesis investigated to which extent Korean heritage speakers (N = 20) maintain their L1 phonology by comparing their perceptual abilities with those of native speakers and English-speaking second language learners of Korean (N = 20) in an AX discrimination task. An AX discrimination task was implemented to measure perceptual accuracy of participants in discriminating the Korean lenis–aspirated stop contrast (aspirated stops: ‘ㅍ’ [p^h], ‘ㅌ’ [t^h], ‘ㅋ’ [k^h]; lenis stops: ‘ㅂ’ [p], ‘ㄷ’ [t], ‘ㄱ’ [k]) which is believed to be the most challenging among the Korean laryngeal categories to acquire not only for second language learners but also native speakers. To investigate whether linguistic factors known to be correlated with overall proficiency of heritage speakers can extend to Korean heritage speakers’ perceptual abilities, the current thesis examined the effects of language use and exposure, age of acquisition, and articulation rate on the perceptual accuracy in the AX discrimination task. Results of a mixed-effects logistic regression model showed that heritage speakers were as accurate as native speakers in discriminating the contrast while outperforming second language speakers. Another finding of the current study is that verbal fluency of heritage speakers measured by articulation rate was found to be a predictor for their perceptual accuracy. The results align with previous work suggesting that heritage speakers have the advantage of early language exposure and use that is sufficient to develop and maintain native-like phonological perceptual abilities later in life.</p>