Two populations inhabiting shanty towns in Rio de Janeiro, Brazil were selected to determine the effectiveness of an ecological control program for human parasitic disease. Following a census and parasite survey both populations were treated with pyrantel pamoate and thiabendazole, this treatment being repeated 10 mo. later. Plants shown to have parasitic inhibitory activity were introduced in 1 area, leaving the other as a control. Of these plants, only lemon-grass, Cymbopogon citratus, survived and reached sufficient density. In the planted area ancylostomiasis levels decreased after 21 mo. from 23.2% to 2.2% and strongyloidiasis decreased from 17.1% to 0.6%. In the umplanted area ancylostomiasis dropped from 14.5% to 5.8% and strongyloidiasis dropped from 13% to 2.9%.