Behavioral studies on bilingual learning have revealed cognitive costs (lower accuracy and/or higher processing time) when the language of application differs from the language of learning. The aim of this functional magnetic resonance imaging (fMRI) study was to provide insights into the cognitive underpinnings of these costs (so-called language-switching costs) in mathematics. Twenty-nine bilingual adults underwent a 4-day arithmetic training in one language, followed by an fMRI test session in which they had to solve the trained problems in both languages. Language-switching costs were accompanied by increased activation in areas associated with magnitude processing (intraparietal sulcus), visuo-spatial imagery (precuneus), numerical stimulus recognition (fusiform gyrus) and executive functions (frontal areas). These findings suggest that language-switching costs in arithmetic are due to additional numerical information processing. Bilingual education programs need to take these findings into account to reduce language-switching costs in order to fully exploit the potential of bilingual learning.