Cycle-to-cycle variations in the period of the vocal fold waveform, known as jitter or frequency perturbation, are a natural quasiperiodic property of the glottal sound source. Excessive jitter in the voice signal represents an undesirable vocal parameter which the singer strives to minimize. This study examined the degree of frequency perturbation in female singers' voices during the production of sung and spoken vowels. Four female singers produced the vowel /a/ in one spoken and four sung conditions. Magnitude perturbation measures were obtained for a steady state portion of each sample. The samples were also submitted to a panel of listeners who were asked to rate the degree of roughness in each vowel. The perceptual results from those judges who performed reliably were compared to the measures of vocal jitter. The results indicate that singers' spoken samples contain less jitter than their sung vowels. The perceptual ratings indicated that listeners were not able to judge the group in relation to their perturbation measures.