Subjects identified concurrent synthetic vowel pairs that differed in relative level and fundamental frequency difference (DF0). Subjects were allowed to report one or two vowels for each stimulus, instead of being forced to report two vowels. At all levels, identification was better at a DF0 of 6% than at unison, but the effect was larger if the target vowel level was below that of the competing vowel. The existence of a DF0 effect when the target was at -10 or -20 dB relative to the competing vowel was interpreted as evidence that segregation occurs according to harmonic cancellation rather than harmonic enhancement. The pattern of identification as a function of level and vowel pair was found to be incompatible with several models of vowel segregation.