Morphological analysis, modelling and topological methods have been used to investigate the influence of a vesicular-arbuscular (VA) endomycorrhizal infection on the root system of Platanus acerifolia, a very common tree species in urban environments. Root systems of endomycorrhizal plants did not differ during the earliest growth period, but at five weeks' growth and onwards the overall effect of mycorrhiza formation was to increase lateral root frequency, giving rise to a more branched root system. During the earliest growth phase, root systems of P. acerifolia developed a herringbone pattern, which then tended towards a more dichotomous pattern in mycorrhizal plants after five weeks when infection was maximum and a mycorrhizal growth response occurred. This study shows for the first time that VA mycorrhizal infection can considerably affect root morphogenesis in a tree species.