Bilinguals need an effective neural mechanism to select and control their languages for successful communication. Recent evidence indicates that the left caudate nucleus (LCN) is a critical part of this mechanism. Here we show that bimodal bilinguals, who use spoken and sign languages, have greater grey matter volume (GMV) in the head of the LCN as compared to monolinguals. We also found higher functional activation of this region in bimodal bilinguals when they switched between sign language and spoken language compared to when they did not switch languages. Furthermore, GMV was positively correlated with the magnitude of the switching effect in the head of the LCN. These findings indicate that for bimodal bilinguals, the LCN is shaped by bilingualism both anatomically and functionally.