Although the antimalarial campaign always had priority, the Serviço de Erradicação de Malária e Profilaxia da Doença de Chagas of the State of São Paulo (Brazil) never neglected, even at the beginning of the campaign, to fight against the triatomid vectors of Chagas' Disease. Because there was no interruption in the prophylaxis, it is not unusual at present to find several localities without triatoma, mainly in the areas where the Triatoma infestans was predominant. This is due not only to the methods of prophylaxis adopted, but also to the socio-economic development of the region. On account of the reduction of the number of foci and density of triatoma the authors found total coverage spraying unnecessary, giving preference to the Selective method. Through a comparative study of costs for the two methods of disinsectization, a mathematical formula was established, for the determination of up to what percentage of infestation in the houses of an area, the selective method, which has been said to be the best, is also the most economical. Knowing the unitary cost of the investigated house (Cp) and the sprayed house (Ce) it is possible to determine the percentage of infested houses (Xe) to which the cost of the work is the same, be it total spraying, be it selective spraying, using the formula Xe = 100 (1- Cp/Ce); to a percentage of houses infested with less than Xe, the Selective method will cost less.
