The study is focused on the elucidation of the role of cytokinins (CKs, zeatin and zeatin riboside) in the development of effective arbuscular mycorrhiza (AM) symbiosis with Medicago lupulina. An important mechanism involved in the regulation of host plant growth is supposed to be linked to the modulation of plant hormone balance. The data obtained revealed the formation of an effective AM-symbiosis (M. lupulina + Rhizophagus irregularis) under phosphorus-deficiency. At the shooting stage (35^th day after sowing), it is characterized by a decrease in the root:shoot ratio, the lowering in arbuscules and vesicle abundances, but an increase in the intensity of mycelium development. Mycorrhized plants differed from the control ones by higher CK levels in both roots and leaves. Zeatin and zeatin riboside concentration exhibited uneven alterations over time. A role of mycelium in the modulation of CK balance has been discussed.