Some listeners exhibit higher sensitivity to subphonemic acoustic differences (i.e., higher speech gradiency). Here, we asked whether higher gradiency in a listener's first language (L1) facilitates foreign language learning and explored the possible sources of individual differences in L1 gradiency. To address these questions, we tested 164 native Spanish speakers with different linguistic profiles. Speech gradiency was assessed via a Visual Analogue Scale task, and foreign language proficiency was assessed via an English vocabulary test. Possible sources of gradiency included domain-general auditory acuity, overall exposure to spoken language (indexed by age), and exposure to phonological diversity. Control measures were collected to account for variables such as phoneme categorization consistency, working memory, and musical training. The results revealed a positive link between L1 speech gradiency and vocabulary acquisition in a foreign language over and above all other variables. L1 speech gradiency itself was predicted by domain-general auditory acuity and overall exposure to spoken language.