The injury of the southern root-knot nematode, Meloidogyne incognita, to a susceptible sweet potato variety, Norin No. 1, was investigated in a field in Kumamoto, Japan, heavily infested with M. incognita. Before planting the cuttings, a part of the field was treated with 320 liters per hectare of nematicide, EDB-EDC (15:40), as a check. At harvest, the number of tuberous roots per plant and their weights were measured and compared between nematicide-treated and untreated plots. The means of number and yield of tuberous roots per plant were increased in the nematicide-treated plots at 5% significant level, but there was no significant difference in the mean weights of tuberous roots between the two treatments. In the following year, without application of nematicide, there were no significant differences in these three characters between the two treatments simply because of the recovery of nematode populations.