The present study reports on the findings of a cross-sectional acoustic study of the production of Spanish vowels by three different groups of speakers: 1) native Spanish speakers; 2) native English intermediate learners of Spanish; and 3) native English advanced learners of Spanish. In particular, we examined the production of the five Spanish phonemic monophthongs (/i e a o u/) in two prosodic positions: stressed and unstressed. Since Spanish and English vary in the nature of the difference between stressed and unstressed vowels, a logical question is: How does the relationship between stressed and unstressed vowels differ across the three groups of participants? Do the learners transfer English patterns to their Spanish? The findings are discussed in terms of their significance for current models of second language speech learning and provide important information regarding the paths of pronunciation development in the second language acquisition of Spanish.