Recent evidence suggests that domain-general auditory processing (sensitivity to the spectro-temporal characteristics of sounds) helps determine individual differences in L2 speech acquisition outcomes. The current study examined the extent to which focused training could enhance auditory processing ability, and whether this had a concomitant impact on L2 vowel proficiency. A total of 98 Japanese learners of English were divided into four groups: (1) Auditory-Only (F2 discrimination training); (2) Phonetic-Only (English [æ] and [ʌ] identification training); (3) Auditory-Phonetic (a combination of auditory and phonetic training); and (4) Control training. The results showed that the Phonetic-Only group improved only their English [æ] and [ʌ] identification, while the Auditory-Only and Auditory-Phonetic groups enhanced both auditory and phonetic skills. The results suggest that a learner's auditory acuity to key, domain-general acoustic cues (F2 = 1200-1600 Hz) promotes the acquisition of knowledge about speech categories (English [æ] vs. [ʌ]). (PsycInfo Database Record (c) 2022 APA, all rights reserved).