Purpose: To determine the relative importance of acoustic parameters (fundamental frequency [F0], formant frequencies [FFs], aperiodicity, and spectrum level [SL]) on voice gender perception, the authors used a novel parameter-morphing approach that, unlike spectral envelope shifting, allows the application of nonuniform scale factors to transform formants and more direct comparison of parameter impact. Method: In each of 2 experiments, 16 listeners with normal hearing (8 female, 8 male) classified voice gender for morphs between female and male speakers, using syllable tokens from 2 male-female speaker pairs. Morphs varied single acoustic parameters (Experiment 1) or selected combinations (Experiment 2), keeping residual parameters androgynous, as determined in a baseline experiment. Results: The strongest cue related to gender perception was F0, followed by FF and SL. Aperiodicity did not systematically influence gender perception. Morphing F0 and FF in conjunction produced convincing changes in perceived gender--changes that were equivalent to those for Full morphs interpolating all parameters. Despite the importance of F0, morphing FF and SL in combination produced effective changes in voice gender perception. Conclusions: The most important single parameters for gender perception are, in order, F0, FF, and SL. At the same time, F0 and vocal tract resonances have a comparable impact on voice gender perception.