A delayed mimicry paradigm was used to assess speakers awareness of non-distinctive phonetic differences which in part distinguish languages. The notion of “phonological filtering” implies that second language learners may not be able to perceive phonetic differences between their native language and a foreign language unless the phonetic differences are linguistically relevant in the native language. If cross-language phonetic differences are in fact perceived poorly, it is unlikely that phonetic modification will occur in the course of naturalistic second language acquisition. In this study native English speakers familiar with Spanish-accented English attempted to read sentences with a Spanish accent. Acoustic measurements showed that two phonetic characteristics of English—the long VOT values associated with /p,t,k/ and final-syllable lengthening—were altered in the direction of Spanish and Spanish-accented English. These results provide tentative evidence that non-distinctive phonetic differences between languages are detectable by language learners and thus do not present an insuperable barrier to phonetic learning in second language acquisition. © 1982, Cambridge University Press. All rights reserved.