A simple method for screening natural products on triatomine bugs was developed using fourth instar Rhodnius neglectus nymphs as an insect model. The statistical method of randomized blocks and randomized blocks with a dichotomous variable were compared and the minimum sample size for screening purposes was established. Tests were made on 94 plant extracts from 49 species, 28 of which are traditionally regarded as having some effect on arthropods or parasitic helminths. 50 mg of crude hexane or ethanol extract was topically applied to each insect. The effect on mortality and moulting was assesed. Ten plant species cuasede caused an increased mortality (16.0 percent to 52.3 precent) compared with untreated controls. Moulting inhibition ranged from 22.2 percent to 33.3 percent and was related to insecticidal effect in most cases. Extracts from two Meliaceae plants belonging to the genera Melia and Cabralea were among the most actived tested, causing 32.5 percent and 45 percent mortality, respectively. Extracts from a Salvia species killed 52.3 percent of the bugs. Ten of the 13 plants showing activity are used by the rural population in various parts of Latin America for arthropod control or as vermifuges