The preverbal double negation construction (i.e., yo "tampoco no" voy a la fiesta hoy) is often cited as a characteristic of Basque Spanish, although its use is undocumented in linguistic studies. The present research aims to fill this void in the literature by analyzing and contrasting the results of two studies on preverbal double negation constructions by Basque Spanish-Basque bilinguals (n = 20) and Basque Spanish monolinguals (n = 20). The first study consisted of an acceptability judgment task, in which the participants rated the acceptability of sentences with a preverbal double negation construction using a 5-point Likert scale. The results revealed that neither Basque Spanish-Basque bilinguals nor Basque Spanish monolinguals from this experiment judged the sentences as acceptable. The results of the second study, which was a semi-structured interview, showed that the monolinguals did not produce constructions containing preverbal double negation. The results also showed that six bilingual participants with a higher dominance in Basque than the group average did produce them. It is proposed that the preverbal double negation construction in their Basque Spanish variety is due to the structural influence of Basque.