We assessed monolingual Spanish and bilingual Spanish-Basque toddlers’ sensitivity to gender agreement in correct vs. incorrect Spanish noun phrases (definite article + noun), using a spontaneous preference listening paradigm. Monolingual Spanish-learning toddlers exhibited a tendency to listen longer to the grammatically correct phrases (e.g., la casa; “the house”), as opposed to the incorrect ones (e.g., *el casa). This listening preference toward correct phrases is in line with earlier results obtained from French monolingual 18-month-olds (van Heugten & Christophe, 2015). Bilingual toddlers in the current study, however, tended to listen longer to the incorrect phrases. Basque was not a source of interference in the bilingual toddler’s input as Basque does not instantiate grammatical gender agreement. Overall, our results suggest that both monolingual and bilingual toddlers can distinguish between the correct and incorrect phrases by 18 months of age; however, monolinguals and bilinguals allocate their attention differently when processing grammatically incorrect forms.