Previous research found that second language (L2) experience may cause an interaction between first language (L1) and L2 categories. This study examines the effect of L2 experience on L1 and L2 stop perception. Three groups of Spanish learners of English varying in L2 experience completed a /p/-/b/ identification task-which included stimuli varying along a Voice Onset Time continuum-in each language. Results suggested that L2 experience improves the perception of L2 stops, but that it also affects L1 perception. Moreover, all groups appeared to have a shared L1-L2 category for bilabial stops, as perception patterns did not differ between languages.