This dissertation examines the possible influence of language learners' second language (L2) on their perception of phonological contrasts in their third language (L3). Previous studies on Third Language Acquisition (TLA) suggest various factors as possible sources of cross-linguistic influence in the acquisition of an L3. This dissertation specifically investigates whether learners' levels of perceptual performance in the L2 is related to their phonological perception of L3 contrasts. In order to examine the perception of Japanese contrasts by non-native learners, I conducted a forced-choice identification experiment (Experiment 1) and AXB discrimination experiment (Experiment 2) with native speakers of English and native speakers of Korean who were learning Japanese at an introductory level. In addition, the Korean participants also participated in a forced-choice English minimal pair identification experiment (Experiment 3). In order to answer the main research question, I examined whether there was any correlation between the Korean participants' perceptual performance in English (L2) and Japanese (L3). There was a positive correlation between the identification of the Japanese word-initial stop voicing contrast and the identification of English minimal pairs. Distinguishing Japanese voiced stops and voiceless stops is widely known to be difficult for native speakers of Korean especially in word-initial position. Therefore, this positive correlation is considered as an indication of a positive influence of learners' L2 on speech perception in their L3. The L2, however, did not influence the perception in the L3 negatively. This result indicates that the participants experienced positive influence from both of their background languages, which supports the idea expressed in the Cumulative-Enhancement Model. Positive correlations were also observed for the discrimination of several other Japanese contrasts and the identification of English minimal pairs. These correlations are considered to indicate an increase in the learners' sensitivity to the speech sounds in general. Different types of correlation results obtained for the identification and the discrimination tasks are considered to reflect the difference in the nature of these perception tasks. All in all, the results suggest that L3 perception is qualitatively different from L2 perception and that the perceptual level in the L2 is related to perception in the L3 at least to some extent. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]