Genetically modified (GM) cotton plants that effectively control cotton boll weevil (CBW)， which is the most destructive cotton insect pest in South America， are reported here for the first time. This work presents the successful development of a new GM cotton with high resistance to CBW conferred by Cry10Aa toxin， a protein encoded by entomopathogenic Bacillus thuringiensis (Bt) gene. The plant transformation vector harbouring cry10Aa gene driven by the cotton ubiquitination-related promoter uceA1.7 was introduced into a Brazilian cotton cultivar by biolistic transformation. Quantitative PCR (qPCR) assays revealed high transcription levels of cry10Aa in both T GM cotton leaf and flower bud tissues. Southern blot and qPCR-based 2 analyses revealed that T GM plants had either one or two transgene copies. Quantitative and qualitative analyses of Cry10Aa protein expression showed variable protein expression levels in both flower buds and leaves tissues of T GM cotton plants， ranging from approximately 3.0 to 14.0?μg?g fresh tissue. CBW susceptibility bioassays， performed by feeding adults and larvae with T GM cotton leaves and flower buds， respectively， demonstrated a significant entomotoxic effect and a high level of CBW mortality (up to 100%). Molecular analysis revealed that transgene stability and entomotoxic effect to CBW were maintained in T generation as the Cry10Aa toxin expression levels remained high in both tissues， ranging from 4.05 to 19.57?μg?g fresh tissue， and the CBW mortality rate remained around 100%. In conclusion， these Cry10Aa GM cotton plants represent a great advance in the control of the devastating CBW insect pest and can substantially impact cotton agribusiness.