The main objective of this research is to demonstrate that the phytohormone ethylene plays a significant role in the formation of arbuscular mycorrhizae (AM). Using improved methods for both the application of exogenous ethylene and quantification of AM fungal colonization, it was shown that a continuous flow of exogenous substrateethylene inhibits colonization of pea (' Pisum sativum') and leek ('Allium porrum') by the AM fungus, 'Glomus aggregatum'. Ethylene (5.5 ppm) reduced colonization of pea by limiting the success of fungal root entry and restricting the extension of colonization units; additionally, the morphology of symbiotic fungal structures was altered. In leek roots exposed to 0.6 ppm ethylene, colonization was reduced; however, morphological alterations were not observed. Treatment with 0.3 ppm ethylene did not significantly affect colonization, but results suggest that lower levels may stimulate AM formation in leek. Ethylene did not affect the number of appressoria formed on roots of either species.