In this article, we sought to investigate the acquisition of gender features by two Basque–Spanish bilingual children when compared to a Spanish monolingual child. Basque is a language that lacks gender features and nominal agreement, whereas Spanish classifies nouns into two classes, that is, masculine and feminine, and has determiner phrase internal agreement. The internal architecture of Basque and Spanish differ on two crucial ways: the presence or absence of agreement and the presence or absence of the syntactic projection ClassP. Hence, the acquisition of gender sheds some light on the internal architecture of the determiner phrase. The studies on gender acquisition by Spanish monolinguals or bilinguals of any combination are not numerous. For this reason, we provide a detailed description of gender development and a thorough analysis of gender errors by a monolingual Spanish child and two Basque–Spanish bilinguals. This study shows that the masculine is not the default gender, neither for the monolingual child analysed nor for bilinguals because both groups overgeneralize masculine as well as feminine. Moreover, none of the children exclusively use the masculine with all nouns at a first stage to converge to target grammar in a subsequent stage. Basque–Spanish bilinguals use masculine determiners with feminine nouns in the majority of contexts from the outset of language acquisition, whereas the monolingual child performs the opposite way. Interestingly, bilinguals require more time to acquire the intricacies of Spanish gender. In other words, they make gender errors even at advanced stages of development, when the monolingual Spanish child studied in this article presents a target-like gender performance. The analysed data show that Basque influences Spanish, resulting in language delay because of the internal architecture of the determiner phrase and to a minor extent by the surface overlap between Spanish and Basque. However, our interpretation is cautious because of the scarcity of such examples and the limited corpus available.