Calluna vulgaris root meristems, either uninfected or infected byPezizella ericae, were compared using light and transmission electron microscopy. In lightly infected ‘hair’ roots, active tips contained initial cells and derivates similar to those of the uninfected roots. The meristem region of most of the heavily infected ‘hair’ roots was strongly reduced. The relationship between the fungus and the root is complex, and the presence of the fungus may influence the organization of the apical meristem.