Stimulus order effects in vowel discrimination were examined. Two experiments were conducted to investigate (l)how the Japanese listeners identify American English vowels and (2) how the Japanese discriminate the English vowel pairs. Five Japanese subjects participated in the experiments. The stimuli were synthesized American English vowels. It was found that stimulus order effects were largest when the stimulus pairs were in the neighborhood of the English vowels that corresponded to Japanese prototype vowels, although the effects were smaller when the stimulus pairs were far from Japanese prototype vowels. The results suggest that stimulus order effects should be determined by the prototype vowels of listeners' native language and by continuum endpoints employed for the experiments.