Subjects identified concurrent synthetic vowel pairs that differed in relative amplitude and fundamental frequency (F0). Subjects were allowed to report one or two vowels for each stimulus, rather than forced to report two vowels as was the case in previously reported experiments of the same type. At all relative amplitudes, identification was better at a fundamental frequency difference (DF0) of 6% than at 0%, but the effect was larger when the target vowel amplitude was below that of the competing vowel (-10 or -20 dB). The existence of a DF0 effect when the target is weak relative to the competing vowel is interpreted as evidence that segregation occurs according to a mechanism of cancellation based on the harmonic structure of the competing vowel. Enhancement of the target based on its own harmonic structure is unlikely, given the difficulty of estimating the fundamental frequency of a weak target. Details of the pattern of identification as a function of amplitude and vowel pair were found to be incompatible with a current model of vowel segregation.