Abstract Previous studies disagree as to whether heritage bilinguals demonstrate loss of knowledge of Spanish grammatical gender. As phonetic variability is known to affect the acquisition of certain grammatical markers, we examine whether bilinguals’ gender difficulties relate to bilingual contact-induced phonetic variability, namely, reduction in the inventory of word-final unstressed vowels. We analyzed narratives from children in the United States (n = 49, ages 4–12). All NP s (n = 1415) were analyzed for structure, noun class, and morphology. Word-final vowels were sub-selected for acoustic analyses. Morpho-syntactically, group results show high accuracy with gender (95%), but with wide individual variation (44%–100%). Speakers also show individual variability and substantive numbers of vowel misclassifications (6%–33%) with higher variability for /a/ and /o/. We found bilingual effects in both domains but no association between phonetics and gender accuracy. These findings have implications for the relationship between phonetics and grammar, and for the morphosyntax of Spanish gender.