The first field trials in Australia of transformed cottons expressing the CryIA(b) insecticidal protein from Bacillus thuringiensis subsp. kurstaki (Bt) were completed during the 1992-93 season. The trials showed good efficacy of the plants against field populations of Helicoverpa armigera, but there were indications of a declining level of Bt expression once plants began to senesce. Laboratory assays showed that the larger instars could survive on the transgenic tissues although their growth was severely retarded. The introduction of Bt transgenic cottons may have several ecological impacts, apart from their direct impact on target pests. These include the risk of resistance development, effects on beneficial and non-target arthropod species and changes in pest status associated with altered patterns of pesticide usage. Chief among the potential pests are sucking insects (e.g. Miridae) which appear not to be regulated by beneficial agents and are currently suppressed by sprays applied for Helicoverpa. Transgenic Bt plants suffered heavily from mirid attack in unsprayed plots. The potential importance of these ecological changes is discussed. Resistance management for transgenic cotton in Australia is likely to depend on a refuge strategy complemented by high expression of Bt protein in the plants, building in the successful implementation of the Insecticide Resistance Management Strategy to ensure compliance with appropriate refuge options. The development and assessment of these resistance management strategies is a key component of pre-commercialization studies in Australia.