• The widespread occurrence of anastomoses and nuclear migration in intact extraradical arbuscular mycorrhizal (AM) networks is reported here. • Visualization and quantification of intact extramatrical hyphae spreading from colonized roots into the surrounding environment was obtained by using a two-dimensional experimental model system. • After 7d the length of extraradical mycelium in the AM symbiont Glomus mosseae ranged from 5169mm in Thymus vulgaris to 7096mm in Prunus cerasifera and 7471mm in Allium porrum, corresponding to 10, 16 and 40mmmm-1 root length, respectively. In mycelium spreading from colonized roots of P. cerasifera and T. vulgaris, contacts leading to hyphal fusion were 64% and 78%, with 0.46 and 0.51 anastomoses mm-1 of hypha, respectively. Histochemical localization of succinate dehydrogenase activity in hyphal bridges demonstrated protoplasmic continuity, while the detection of nuclei in the hyphal bridges confirmed the viability of anastomosed hyphae. • The ability of AM extraradical mycelium to form anastomosis and to exchange nuclei suggests that, beyond the nutritional flow, an information flow might also be active in the network.