The activity of cell-wall-bound peroxidase in Allium porrum L. roots was measured during root growth and development of vesicular–arbuscular mycorrhiza with Glomus versiforme (Karst.) Berch. The peak of activity in infected plants was higher than in non-infected ones and occurred during the initial stages of fungal penetration. When the infection was established and the roots highly colonized the activity decreased to the levels in non-mycorrhizal roots. Ultrastructural localization using diaminobenzidine revealed an increase in the electron-dense deposits in the middle lamella corresponding with fungal penetration. No activity was seen in the matrix deposited around the intracellular fungal structures. The overall distribution of peroxidase activity was the same in mycorrhizal and non-mycorrhizal roots.