Numerous studies on the effect of the nectariless gene on insect pests of cotton have been carried out both overseas and in Australia. However most of the latter studies have been conducted under small plot conditions which are not really suitable for assessing the effects of this character. This trial measured pest populations on large plots of normal and nectariless cotton isolines grown under standard commercial management practices. It was also designed to study anv possible differences in the ovipositional responses of the two Heliothis spp. (H._ armigera and H .punctigera) to these lines: an area of research which had been neglected in previous studies.