This study analyzes gender assignment in Spanish–Basque mixed nominal constructions with nouns in Basque (a language that lacks gender) and determiners in Spanish (a language that marks gender) by using a multi-task approach: (i) naturalistic data, (ii) an elicitation task, and (iii) an auditory judgment task. Naturalistic data suggest cross-language effects under which a morphological marker of Basque (-a determiner) is interpreted as a morphophonological expression of gender marking in Spanish. A preference for feminine determiners was observed in the judgment task, which differs from the masculine default trend observed in Spanish–English bilinguals (Jake, Myers-Scotton & Gross, 2002). Our results point to feminine gender as default in Spanish–Basque mixed DPs, indicating that the resources that bilinguals use for gender assignment can be different from those of monolinguals. We argue that this is an outcome of interacting processes which take place at the interfaces (lexicon, phonology, morphosyntax) of both languages, resulting in cross-language effects.