With the appearance of pvrethroid resistance in Heliothis armigera during the 1992/3 cropping season a strategy aimed at limiting the development of widespread resistance was rapidly implemented .H. armigera had previously shown a propensity to a acquire high levels of resistance to other insecticides (e.g. DDT). As part of a number of studies on the genetics and ecology of pvrethroid resistance the work described here was commenced to evaluate the biological fitness of resistant individuals relative to their susceptible counterparts.