Four years of light-trap and crop survey data from the Namoi Valley of New South Wales are presented for Heliothis armigera (Hb.) and H. punctigera Wllgr. These suggest there are four generations a year. The associated seasonal sequence of major host-plants supporting successive generations, appeared to be: H. armigera—wheat; pre-flowering cotton; flowering cotton and sorghum; sorghum, flowering cotton and sunflowers; and H. punctigera—lucerne, linseed and medics; pre-flowering cotton; flowering cotton, sunflowers and soyabeans; lucerne, linseed and medics; pre-flowering cotton. On host-plants common to both species, a change in dominance from H. punctigera to H. armigera occured as each summer progressed. Suggested causes are: seasonal changes in the availability of host-plants, favouring H. armigera over H. punctigera; insecticide resistance in H. armigera; and the competitive superiority of H. armigera. Large numbers of H. armigera in cotton appeared to be related to: warm springs, which favoured moth emergence during the period of wheat anthesis; the incomplete control of infestations within cotton itself, leading to the development of resident populations; and the influx of moths from other host-plants, especially sorghum, during the latter half of the season.