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Leaf variation was examined in two hybridizing Mexican red oaks, Quercus affinis and Q. laurina. Data of nine traits were obtained for ten randomly chosen leaves in each of 512 individuals from 16 populations sampled along a geographical gradient, including the distribution area of both species and a putative area of secondary contact and hybridization. A cluster analysis recognized two main groups of populations, which were congruent with the taxonomic assignment of the predominant morphological type within the populations and were thus labelled ‘Q. affinis-like’ and ‘Q. laurina-like’ population groups. A nested analysis of variance revealed that the largest proportion of the total variation was contained within populations, as among-tree variation (28–54%, depending on the trait), and as intra-individual variation (17–56%). However, differences between the two groups of populations (3–27%) and among …