Recent reports on perception and production of speech segments in simultaneous bilinguals, who acquired both of their languages at the same time‐frame (from birth), suggest that they do not always exhibit abilities similar to those of the native monolinguals of the same languages [i.e., Guion (2003); Molnar et al. (2009); Sundara and Polka (2008)] despite the fact that simultaneous bilinguals are native users of both of their languages as well. As opposed to the second language users, who often assimilate the acoustically similar non‐native phonetic categories to their native ones, simultaneous bilinguals rather tend to dissimilate the phonetically similar sounds that occur across both languages. In order to understand the mechanism underlying the phonemic category formation of simultaneous bilingual language users across their two languages, an unsupervised formation of the phonetic perceptual space of English‐French bilinguals was modeled by training self‐organizing maps (SOMs) with vowels from both languages at the same time. The formation of phonetic categories in monolinguals by training SOMs with vowels from French or English only was modeled as well. Differences in the perception of speech segments in the bilingual SOMs with respect to the monolingual SOMs will be discussed and compared to experimental findings.