The view that individual speaker characteristics reflect morphological and functional differences in the speech apparatus has been accepted as self-evident despite the lack of convincing empirical data. A recent study on the issue of individual differences based on x-ray microbeam data from ten American English and ten Japanese speakers (five males and five females in each group) has pointed to a significance of morphology in accounting for speaker characteristics of vowel gesture. The midsagittal form of a speaker's orofacial structure was parametrized by several landmarks on x-ray scan images. A few points and lines were chosen to form a quadrilateral framework so that they define a geometrical ``articulatory space'' where articulation of the tongue--jaw complex takes place. Pellet data for vowels were mapped onto speaker's articulatory space to observe the effect of morphological difference on vowel articulation. Preliminary results have shown the extent of morphological variation of the speech apparatus and adaptability of vowel gesture to different forms of the space. The aspect ratio of the articulatory space was judged as the primary causal factor of articulatory variation. Acoustic manifestation and perceptual contribution of the variation will be discussed from a viewpoint of auditory-articulatory linkage.