Short-term effects of benomyl on the arbuscular mycorrhizal fungus Glomus caledonium (Nicol. & Gerd.) Trappe and Gerdeman associated with Cucumis sativus L. were studied by measuring effects on fungal P transport and on fungal alkaline phosphatase activity. Mycorrhizal plants were grown in three compartment systems where nylon mesh was used to separate n root-free hyphal compartment (HC) and a root + hyphal compartment(RHC) from The main root compartment (RC). Non-mycorrhizal control plants were grown in similar growth units. After 6 wk benomyl was applied to the plants in three ways: as soil drenches to RHC or HC, or as u spray to the leaves. Benomyl was added in three concentrations. Equal amounts of ³²P and ³³P were added to the HC and to the RHC respectively, immediately after the application of benomyl. Plants were harvested 4–6 d later. Hyphal transport of ³²P from the HC was inhibited when benomyl was applied to the HC at 10 µg g^-1 soil, whereas the uptake of ³²P from RHC I roots + hyphae) was reduced only at the highest dose of application to the RHC (100 µ g g^-1 soil). In contrast to the marked reduction of benomyl on fungal P transport, the activity of fungal alkaline phosphatase inside the roots was unaffected by benomyl.