The relationship in geographical distribution and morphological variation of leaflet width and length (diagnostic trait), between and within populations of Dioon edule Lindl., has been investigated throughout its known range in eastern Mexico (from the states of Nuevo León to Veracruz, north to south, respectively). A total of 1832 leaflets were measured for width and length from 154 plants distributed amongst five populations using four leaflet replicas from each of three leaves per plant. For leaflet width and length the variation among populations indicated significant stat-istical differences (F4,147 = 125.83; P < 0.0001; R2 = 92.17% and F4,147 = 9.04; P < 0.001; R2 = 26.8%), respectively. With respect to leaflet width, the multiple range test showed three groups with a north to south distributional relationship along the range of the species. The correlation coefficient among paired populations, respect to geographical distance and the absolute value of the mean difference of leaflet width in each population, was positive, and different from zero (r = 0.82; P = 0.013). A great variation of important ecological and evolutionary parameters was shown. © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 141, 465–470.