Recent research on language production suggests that bilinguals shift from using inhibitory control mechanisms to a language-specific selective mechanism during development (Costa, Santesteban, & Ivanova, 2006). Costa et al. argue that the robustness of the L2 lexical representations may be critical to the functionality of a language-specific selective mechanism. Accordingly, the present study measured the lexical robustness of a group of 54 English dominant learners of Spanish using a verbal fluency task and investigated its effect on their performance in a picture-naming task with language switches. The results suggest that L2 lexical robustness predicts the shift to a language-specific selective mechanism during speech production. Moreover, it is demonstrated that a specific threshold of lexical robustness is necessary to engage the mechanism.