Nematode population densities and yield of sweet potato and onion as affected by nematicides and time of application were determined in a 3-year test. Population densities of Meloidogyne incognita race 1 in untreated plots of sweet potato increased each year, but Helicotylenchus dihystera and Criconemella ornata did not. Ethoprop (6.8 kg a.i./ha) incorporated broadcast in the top 15-cm soil layer each spring before planting sweet potato reduced population densities of nematodes in the soil and increased marketable yield in 1982, but not in 1983 and 1984. When DD, fenamiphos, and aldicarb were applied just before planting either sweet potato or onion, nematode population densities at harvest were lower in treated than in untreated plots. No additional benefits resulted when the nematicides were applied immediately before planting both sweet potato and onion. Correlation coefficients (P = 0.05) between yield of marketable and cracked sweet potato storage roots vs. densities of M. incognita juveniles in the soil at harvest among years ranged from r = -0.33 to r = -0.54 and r = 0.31 to r = 0.54 (P = 0.05), respectively. Temperatures of -6 to -8 C in December 1981 and - 11 to - 13 C in December 1983 killed the onion crops. Correlation coefficients for marketable yield of onion seeded in 1982 and harvested in 1983 vs. densities of M. incognita juveniles and H. dihystera in the soil at harvest were r = -0.42 and r = -0.31 (P = 0.05), respectively. Key words: aldicarb, Allium cepa, chemical control, crop rotation, DD, ethoprop, fenamiphos, Ipomoea batatas, Meloidogyne incognita, root-knot nematode.