The nematicidal activities of ammonia-releasing and ammonium compounds were tested against the root-knot nematode Meloidogyne javanica in pot, microplot and field experiments. Among ten compounds tested, NH4OH, (NH4)2HPO4 and NH4HCO3 showed greatest nematicidal activity at concentrations of 300 mg N/kg of NH4 or NH2 in pots. NH4OH was found to be the most nematicidal of these compounds. Enclosure of pots containing NH4OH-treated soils in plastic bags reduced the concentration of NH4OH needed to kill the nematodes from 200 mg N/kg in open pots to only 75 mg N/kg. In a microplot experiment, treatment of nematode-infested soil with NH4OH at 70 mg N/kg reduced the root-galling index to 0. In one field experiment, the nematicidal efficacy of NH4OH on tomato plants at doses of 1000 and 2000 kg N/ha was equivalent to those of Telopic C35® or metham sodium in combination with cadusafos. In another field experiment, NH4OH at 500 and 1000 kg N/ha increased tomato yield and at 1000 kg N/ha reduced the galling index, compared with untreated controls. The results suggest that NH4OH may serve as a nematicide in alkaline sandy soils.