In laboratory testing, egg hatch of Tylenchulus semipenetrans was stimulated at concentrations of 1 and 10 microg/ml aldicarb solution and inhibited at 50 and 100 microg/ml. Aldicarb was more inhibitory to egg hatch than the aldicarb sulfoxide and the aldicarb sulfone. Inhibition of hatch at the high concentration was associated with delays in the molting processes, lack of larval movement within the egg, and delays in embryonic development. Nematode motility was reduced at 10, 50, and 100 microg/ml of aldicarb and aldicarb sulfoxide solution, and at 50 and 100 microg/ml aldicarb sulfone. Male development was retarded at 10 microg/nrl and almost completely inhibited at 50 and 100 microg/ml of the three chemicals. In greenhouse tests, female development antl reproduction on roots of citrus seedlings were suppressed by aldicarb at rates of 2.6 microg/ml and completely inhibited at 10.6 microg/ml of soil solution during a 50-day experimental period. Under field conditions, there was little systemic movement of aldicarb into roots located outside treated areas. Aldicarb reduced the nematode larvae and the female adult population in the second year after the second treatment. There were no differences in egg hatch and sex ratio of citrus nematodes between treated and nontreated roots.