In Brazil and other developing countries, theprocess of land occupation resulting from the pre-vailing economic development model and the ex-isting social inequalities superimposed on a terri-torial base have produced a major impact on theepidemiological profile of endemic diseases.The introduction of schistosomiasis in Brazilhas been historically attributed to the arrival of thefirst African slaves in the mid-16th century. Fromthe main gateways they were taken to the sugarcane cultivation in the coastal area of northeasternBrazil, where ecological conditions werefavourable for establishing schistosomiasis. Fromthere, the endemic area gradually expanded north-wards and southwards with other agricultural crops.The district of Santa Cruz is located in the westarea of Rio de Janeiro and in order to better under-stand the occupation of this area it is important tohighlight certain relevant historical aspects. Refer-ring once again to the Colonial Period of Brazil,the arrival of the first Jesuit missionaries in the late16th century gave rise to farming and cattle-rais-ing in the area, previously considered inadequatefor such economic activities because of its pecu-liar peat bog terrain, prone to flooding. These natu-ral difficulties were overcome with the develop-ment of an efficient drainage system includingdikes, ditches, and canals. Economic growth inSanta Cruz lasted until 1760 when the Jesuits werebanned from Brazil.In the last four decades it was observed a con-tinuous migration into the Santa Cruz IndustrialDistrict, with no adequate land use planning, thesilting-in of ditches and canals, a phenomenon thatactually began when the Jesuits were expelled fromthe area, became worse in the recent years of in-dustrial development, finally leading to a severeflood of the entire Santa Cruz Lowland during theheavy rains in the summer of 1996.With these heavy rains came an outbreak ofleptospirosis and fieldwork teams from the Brazil-ian National Health Foundation (FNS) were calledinto the area. While these FNS teams were in thearea they noticed an unusually large number ofsnails in both the existing water bodies and floodareas, including peridomiciliary areas.Based on the above-mentioned observation, webegan collaborating with the FNS in studies tomonitor the presence of individuals infected withSchistosoma mansoni, the existence of autochtho-nous cases, and the identification of the intermedi-ate host species, as well as the positive infectionrate in the latter.The first step was to search for the possibletransmission area. This was done by examining arelative small number of faecal specimens from asuspected area where 200 individuals were exam-ined and three were found positive for S. mansoni.The limits of the study area were fixed to in-clude the areas with the greatest concentration ofrecent land invasions, a quadrangle of which theborders were the Ita, Goiaba, Pau da Flecha andVala da Goiaba canals.In initial parasitological survey a sample of thepopulation were selected and school-children fromthree local schools were chosen. The work alsoinvolved setting up a field laboratory in one of thepublic schools.From the stool examination of 1929 school-children, the following helminth prevalence wasfound: 2% for S. mansoni, 56% for Ascarislumbricoides, 32% for Ancylostoma sp., and 55%for Trichuris trichiura.A study on snails population dynamics was alsoperformed with monthly captures in all the waterbodies, both perennial and seasonal. In the latterthe captures were performed until the breeding