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The vocal tract resonances of trained soprano singers were measured while they sang a range of vowels softly at different pitches. The measurements were made by broad band acoustic excitation at the mouth, which allowed the resonances of the tract to be measured simultaneously with and independently from the harmonics of the voice. At low pitch, when the lowest resonance frequency R1 exceeded f(0), the values of the first two resonances R I and R 2 varied little with frequency and had values consistent with normal speech. At higher pitches, however, when f(0) exceeded the value of R I observed at low pitch, R I increased with f(0) so that R I was approximately equal to f(0). R2 also increased over this high pitch range, probably as an incidental consequence of the tuning of R1. R3 increased slightly but systematically, across the whole pitch range measured. There was no evidence that any resonances are tuned close to harmonics of the pitch frequency except for R 1 at high pitch. The variations in R I and R 2 at high pitch mean that vowels move, converge, and overlap their positions on the vocal plane (R 2,R I) to an extent that implies loss of intelligibility.