Bilinguals need control mechanisms in order to switch between languages in different communication contexts (Green, 1998, Bilingualism: Language and Cognition, 1; Price, Green, & von Studnitz, 1999, Brain, 122). There has been neural evidence showing competition to control output in L2 vs. L1 in both cortical and sub-cortical areas, when language selection is carried out (Abutalebi & Green, 2007, Journal of Neurolinguistics, 20). Here we use intra-operative direct electrical stimulation to demonstrate that the head of the left caudate is critical not only in language switching tasks but other control tasks. A bilingual Chinese-English patient was instructed to perform both language switching and switching in color-shape naming tasks during awake glioma surgery. When stimulation was applied on the left caudate, failures or difficulties in both language switching and color-shape naming were observed, with the effects greater on language switching. Stimulation to neighboring brain regions either did not affect performance or generated mild problems specific to language switching. The results provide direct evidence of the necessary role of the left caudate in language control.