The current study investigated the nature of second language (L2) learning influence on first language (L1) production and perception by systematically examining several L1 and L2 related factors (L1:L2 relative dominance, L2 production and perception acuity, and learning context) and their effect on L1 change. It is the first of its kind in examining L1 drift in two different learning contexts, with a relatively large sample size, and including both production and perception. The study recruited 65 late L1-Japanese learners of L2 English whose L1 Japanese and L2 English perception and production of voiceless stops and target vowels were compared to monolingual control groups (Japanese and English). Our data show L1 VOT as well as vowel drift in both perception and production. The production data exhibited cases of both assimilatory and dissimilatory L1 drift, supporting the view that late L2 learners' L1:L2 phonetic systems are dynamic and malleable. We identified L2 production acuity as a major factor associated with L1 assimilatory drift and also demonstrated a complex interplay between L2 production acuity, L2 perceptual ability, and L2 dominance that together appear to influence the direction of the L1 drift. [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.]