Several studies have shown that concepts spread activation to words of both of a bilingual's languages. Therefore, a central issue that needs to be clarified is how a bilingual manages to restrict his speech production to a single language. One influential proposal is that when speaking in one language, the other language is inhibited. An alternative hypothesis is that bilinguals focus only on the language that is relevant for communication. Here these proposals were tested in a series of experiments in which Spanish-Catalan bilinguals named pictures. Cumulative semantic interference (CSI) was used as a window into lexical processing and cross-linguistic interactions. Results revealed that CSI is present between languages with the same magnitude as within-languages. This result cannot be accounted for by any of the above-mentioned models without substantial modifications. Instead, they are suggestive of bilingual processing dynamics qualitatively similar to those of monolinguals. (Contains 9 tables and 7 figures.)