The previously reported activity of natural products of plant origin which inhibit the external larval evolution of Ancylostomidae and of Strongyloides stercoralis, has now been applied in the pratical ecological prophylaxis of human parasitic disease. The study was made in two populations of low economic level inhabiting shanty towns on the Ilha do Governador, Rio de Janeiro, Brasil. During 26 months the following phases were completed: 1 - selection and census ofeach area; 2 - parasitological survey; 3 - mass treatment of both populations with the anthelminthics pyrantel pamoate and thiabendazole, this treatment repeated 10 months later; 4 - introduction of plants with parasitic inhibitory activity in one area, leaving the other as control; 5 - three epidemiological Controls, one after each treatment and then at intervals of 60 to 70 days to determine the prevalence and reinfection leveis of the two parasites in each area. Of the plants introduced only lemon-grass, Cymbopogon citratus, survived and attained an average density of one dump per 10 m². Ancylostomiasis levels fell in the plantedarea from 23.2% to 2.2% and in the control are from 14.5% to 5.8% during the twenty-one month study. Strongyloidiasis fell from 17.1% to 0.6% in the planted area, and from 13% to 2.9% in the control area. 46% of the initially examined populations were followed through to the end. The reduction in prevalence of hookworms, 30.5% higher in the planted area, is considered statistically significant, and the method therefore valid and viable. Supporting evidence is being sought in a parallel experiment with animais where complete control is possible. The method, apparently novel, seems of value in the reduction of reinfection after mass treatment in endemic areas. It is popularly acceptable, cheap and produces quick results and may bridge the gap until definitive measures such as sanitary education and engineering produce their longer term solution. In addition to the medicai and sanitary application, the results presented suggest applications in the veterinary and phytosanitary fields.