A direct spraying method for establishing insecticide dosage-mortality baselines and for monitoring variations in the susceptibility to insecticides of Aemolamia varia saccharina, the major pest of sugar-cane in Trinidad, is described and the experimental errors associated with the method are quantified. Apparent differences between the susceptibilities of male and female froghoppers are related to the different weights of the sexes. Considerable natural variations in susceptibility occur within and between the four annual generations of froghopper· in particular, the second generation may have susceptibilities about twice those of other broods. Base-line data are given for carbaryl, Malathion, monocrotophos and propoxur which are or have been in commercial use in Trinidad and for dioxacarb, omethoate and vamidothion which are expected to be used. The development of resistance to carbaryl is described; the ratios between the 4 h LC50 values of resistant and susceptible portions of field populations varied from 44 to 135 and the proportions of carbaryl-resistant insects from 27 to 58 per cent. Froghoppers resistant to carbaryl were sometimes cross-resistant to propoxur which had not then been used commercially in Trinidad. Resistance values for propoxur ranged from 22 to 112 and the proportion of resistant insects from 15 to 35 per cent. Cross-resistance to dioxacarb was also demonstrated. Carbaryl-resistance in some instances modified the response of froghoppers to malathion, monocrotophos and phosmet in that dosagemorlality lines of unusually low slope were obtained. Only one incidence of Malathion resistance was demonstrated even though the compound has been in use for some years.