Petroleum spray oils (PSOs) have been used for many decades to control a wide range of crop pests (Beattie et al 1995) and are known to have little impact on natural enemies of crop pests (Mensah et al 1995). They form an essential part of many integrated pest management (IPM) programs (Beattie and Smith 1997). Despite these benefits, the use of PSOs in the Australian cotton industry has been limited due to a perceived risk of PSO-induced phytotoxicity. In addition, since PSOs do not have quick knockdown effect like synthetic insecticides, growers do not consider PSOs as appropriate products to use against major cotton pests such as Helicoverpa spp. when their economic threshold is reached. However, historical research has shown that the risk of PSO-induced phytotoxicity can be minimised when a number of key base oil properties are considered in good practice PSO formulation. According to Johnson (1994) the use of a high quality base oil of no less than 91% unsulfonatable residues will ensure that few unsaturated compounds remain in the base oil to cause phytotoxicity. Subsequently, research on citrus and a range of other horticultural crops has led to the development of new PSO formulations, some containing UV light absorbers to eliminate photo-oxidation to further reduce the risk of phytotoxicity. Furthermore, there has been increasing evidence that PSOs similar to summer spray are appropriate for use in cotton to reduce numbers of Helicoverpa spp. eggs and to suffocate larvae (Mensah et al 1995; Liu and Stansly 1995). The insecticidal efficacy of PSOs is related to their viscosities (Johnson, 1994; Beattie at al 1995; Rae at al 1997 and Lui et al 2001). As a result there is a need to undertake studies using high viscosity oils for the management of Helicoverpa spp. on commercial cotton crops. The aim of this study was to determine the efficacy of high viscosity PSO and other crop oils as a stand-alone insecticidal product for activity against Helicoverpa spp. on cotton.