the effectiveness of three operational strategies for the control of triatoma dimidiata was compared by a field trial in the department of madriz, nicaragua. one strategy involved full pretrial evaluation, followed by spraying of all houses irrespective of whether or not they had been found to be infested. the second strategy minimised the pretrial evaluation by considering the locality infested as soon as one house was found to be positive, followed by spraying all houses. the third strategy involved full pretrial evaluation, followed by spraying only those houses found to be positive. evaluation after twelve months indicated that all three strategies were similarly effective, since all sprayed houses remained free of infestation. however, comparative estimates of the unit intervention costs indicated that strategies 1 and 2 were substantially less efficient than the third strategy of spraying only positive houses.