In this paper I claim that there is evidence of grammatical interference in the development of ergative case in bilingual children acquiring Basque and Spanish. While both monolingual and bilingual children have difficulty acquiring the ergative case in Basque (Barrena, 1995; Ezeizabarrena, 1996), my results indicate that bilingual children omit the ergative case marker to a greater extent than monolingual children. I argue that this difference is the result of interference from Spanish, which takes the form of reinforcing a non-target-like option in Basque (Muller and Hulk, 2001).