A 12-week greenhouse experiment was undertaken to test the efficiency of inoculation of vesicular-arbuscular mycorrhizal fungi on four apple (Malus domestica Borkh) rootstock cultivars: M.26, Ottawa 3 (Ott.3), P.16, and P.22. The plants were grown in soil from an apple rootstock nursery, containing high levels of extractable P (644 kg Bray/1 ha-1). Inoculation treatments were Glomus aggregatum Shenck and Smith emend. Koske, G. intraradix Shenck and Smith, and two isolates of G. versiforme (Karsten) Berch, one originally from California (CAL) and the other one from Oregon (OR). Mycorrhizal plants were taller, produced more biomass, and had a higher leaf P concentration than the uninoculated control plants. Mycorrhizal inoculation also significantly increased the leaf surface area of `M.26' and `Ott.3' compared to the control. Glomus versiforme(CAL)-inoculated plants generally had the best nutrient balance, the greatest final height and shoot biomass, and produced an extensive hyphal network. All the mycorrhizal plants had similar percentages of root colonization, but the size of the external hyphal network varied with fungal species. Glomus versiforme(OR) had a larger extramatrical phase than G. aggregatum and G. intraradix. Mycorrhizal efficiency was associated with a larger external hyphal network, but showed no relation with internal colonization. Despite the high P fertility of the soil used, growth enhancement due to mycorrhizal inoculation was attributed to improved P nutrition.