In beech seedlings during their first growing season, both in pot experiments and in natural woodland conditions, the root tips passed through a developmental sequence in which the elongation rate progressively declined, root diameter decreased, the root cap became smaller, and tissue differentiation approached nearer to the apex. Such a spatially precocious differentiation is characteristic of mycorrhizas, where it has generally hitherto been regarded as a consequence of fungal infection, but in these seedlings it was a normal developmental change associated with progressive decline in activity of the root meristem, and was independent of the presence of mycorrhizal fungi. Indeed, mycorrhizal infection occurred only when a root tip had passed through this sequence and elongation had nearly or completely ceased. The formation of mycorrhiza caused a resumption of slow elongation, and a constriction in the root marked the position of the root tip at infection: the mycorrhizal region was swollen owing to cortical cells being expanded in the transverse plane. Certain features of the structure of the root of the host varied quantitatively with the species of mycorrhizal fungus. An attempt is made to identify those aspects of mycorrhizal structure which resulted from normal root development and those which were induced by the fungus.