The stem and bulb nematode, Ditylenchus dipsaci, is a plant parasite that causes significant economic losses to growers of the main host plant, onion. Fungal mycorrhizae can increase plant growth and induce systemic resistance against many diseases and pests. However, no evidence of a detailed study was found regarding the relationship of mycorrhizae with D. dipsaci. In this study, the effects of the mycorrhizae of the fungus, Rhizophagus irregularis, on D. dipsaci, the stem and bulb nematode, on the onion plant, Allium cepa, were investigated. The mycorrhizae did not significantly reduce nematode penetration and multiplication on onion plant roots. Mean penetration rates for mycorrhizal and non-mycorrhizal plants were 13.5% and 7.5%, respectively. The multiplication rate was between 0.6 and 1.3 in a growth chamber and 0.7 and 3.6 in a greenhouse. Fresh plant weight was 0.9-2.2 g in the greenhouse. Since the mycorrhizae of R. irregularis increased the growth of the onion plant by increasing its tolerance to D. dipsaci, it would be beneficial to increase mycorrhizal levels in onion growing areas where D. dipsaci, the stem and bulb nematode, is present.