The use of carbon isotopes as a 'natural marker' of Helicoverpa has proven very useful in this regard (see below). This short paper presents some preliminary results of carbon isotope analyses that we have conducted using both H. armigera pupae collected within a range of different crops and reared to adulthood, and mating moths collected within Bt crops. The research is a follow on from a preliminary study by G. Fitt and P. Gregg (unpublished data) who first recognised the potential for using carbon isotope signatures for identifying the plant host origins of Helicoverpa moths in Australia. Others (e.g. Gould et al 2002) have used the technique to identify plant host origins for other moths overseas.