A body of work over the past several decades has demonstrated that the complex and coordinated articulatory movements of human vowel production are governed (at least in part)by control mechanisms whose targets are regions of auditory space. Within the target region control at the sub-phonemic level has also been demonstrated. But the degree of accuracy of that control is unknown. The current work investigates this question by asking how far apart must two vowel stimuli lie in auditory space in order to yield reliably different imitations? This distance is termed 'Just Producible Difference' (JPD). The current study uses a vowel mimicry paradigm to derive the first measurement of JPD among two sets of English speakers during front vowel production. JPD is estimated at between 14 and 51 mels in F1 X F2 space. This finding has implications for episodic theories of speech production. It also clarifies the possible structures of human vowel systems, by setting a theoretical lower bound for how close two vowel phonemes may be in a speaker's formant space, and hence a psychophysical explanation of observed trends in number and patterns of possible vowel phonemes.