This study is divided into three main parts. The first is an investigation of the effect of vesicular - arbuscular mycorrhizal fungus Endogone on the growth of Allium cepa and Allium porrum. Onion and leek seedlings were inoculated with Endogone spores and the progression of the mycorrhizal colonisation followed. The effects of the mycorrhizal association on the plants were determined by measuring the shoot and root dry weight, number of roots produced and height of plant. Plants in which mycorrhiza became well established showed significantly enhanced growth rates. The second part of this study was an investigation on the possible role of vesicular - arbuscular mycorrhiza in the resistance of onion plants to two soil – borne fungal diseases, namely basal root rot, caused by Fusarium oxysporum var. cepa and pink rot, caused by Pyrenochaeta terrestris. A marked resistance to these pathogens was found in plants with a well developed mycorrhizal association. An increased phosphate nutrition of the plants provided by applying CaHPO4.2H2O to soil media was found to be insufficient to afford disease resistance. The third part of this investigation was an attempt to culture Endogone in agar media in the presence of A. cepa, using low phosphate concentrations of CaHPO4.2H2O, calcium phytate, sodium phytate or lnositol. Germination of Endogone spores in the same phosphate sources containing the vitamins nicotinic acid and thiamine HCI was also investigated. Calcium phytate elicited the best response in the culturing of Endogone in agar media, while the vitamins, used together and in combination with calcium phytate gave the highest percentage germination of Endogone spores. An ultrastructural study of vesicular – arbuscular mycorrhizal development was carried out to determine the spatial relationships between the endophyte and host cells. Difficulty was encountered with the fragility of the root tissue.