In Experiment 1 categorial discrimination tests. Thai bilinguals were able to discriminate stop contrast differing in place of articulation in their two languages. But English-speaking listeners’ discrimination was good only for English. English stops produced by native Thai speakers were perceived most accurately by both English and Thai listeners. There appeared to be a positive cross-language transfer effect when Thai listeners heard Korea stimuli. Thai bilinguals were a homogeneous group with respect to their patterns of stop perception. This suggests robustness of L1 perception system in adult bilinguals.