Previously [R. Colton and H. Hollien, J. Acoust. Soc. Amer. 47, 105(A), (1970)] it was reported that observers could distinguish modal and falsetto voice quality when the phonations from the two registers were presented at the same fundamental frequency and intensity. Other data [R. Colton, (unpublished)] demonstrates that modal phonations exhibit more energy in the higher harmonics than do falsetto phonations; suggesting that frequency bandwidth is an important cue for the differentiation of modal and falsetto voice quality. Phonations produced in the two vocal registers were low-pass filtered to equate the bandwidth of modal phonations to falsetto phonations. The filtered phonations were presented in a paired comparison paradigm to observers who judged the similarity or dissimilarity of voice quality of each stimulus pair. The proportion of correct responses was 0.71; a decrease of 20% when compared with the responses obtained for these phonations in an unfiltered condition. These results indicate that stimulus bandwidth is an important acoustic parameter for the differentiation of the modal and falsetto registers. However, an analysis of individual phonations suggest other acoustic differences may aid in the distinction of modal and falsetto voice quality.