The perceptual organization of speech remains poorly understood. Recent research using sine-wave speech suggests that the ability of an extraneous formant to impair intelligibility depends on modulation of its frequency contour [Roberts et al., J. Acoust. Soc. Am. 128, 804–817]. This study examined the effect on intelligibility of manipulating the depth of this frequency variation. Three-formant (F1+F2+F3) analoges of natural sentences were synthesized using a monotonous glottal source (F0=140 Hz). Each formant-frequency contour was scaled to 50% depth about its geometric mean; this manipulation had relatively little impact on intelligibility. Perceptual organization was probed by presenting stimuli dichotically (F1+F2C; F2+F3), where F2C is a competitor for F2 that listeners must resist to optimize recognition. Different competitors were created by inverting the frequency contour of F2 about its geometric mean and varying its depth (100%-0%, 25% steps). Adding F2C typically reduced intelligibility; this reduction was greatest for 100%-depth, intermediate for 50%-depth, and least for 0%-depth (constant) F2Cs. These results indicate that competitor efficacy depends on overall depth of frequency variation, not depth relative to that of the other formants, and suggest that frequency-contour modulation influences across-formant grouping not only in sine-wave analogues but also in more speech-like simulations. [Work supported by EPSRC.]