In aquatic natricine snakes, piscivorous species tend to have narrow streamlined heads, while species that prey on frogs have broader heads. This pattern is thought to reflect the antagonistic design requirements of fast underwater striking on the one hand and the consumption of bulky prey on the other. Here we test whether a similar correlation between head shape and diet exists at the intraspecfic level, by quantifying head shape and diet in the frontal striking aquatic natricine snake Natrix tessellata. Our results show that museum specimens with fish in their stomachs had significantly narrower and more streamlined heads than individuals with frogs in their stomachs. Given that diet is strongly determined by local abundance of potential prey, these results suggest strong population-level divergence in head size and shape in this species. Future studies need to establish whether the observed differences in head shape have a genetic basis, or result from phenotypic plasticity.