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Abstract: Bacillus thuringiensis (Bt) transgenic cotton has been planting in large-scale for more than 10 years in China. It was observed that resistance to bollworm was reduced with the application of big-boll cultivars in China. The objective of the study was to investigate the effects of boll size on the contents of CryIA insecticidal protein in boll shell and cotton seed during the course of boll development. Two experiments were conducted at the Yangzhou University Farm, Yangzhou, China. In 2004, three cultivars including Sikang 1 (small-boll), Xiangza 3 (medium-boll) and Kemian 3 (big-boll) were compared for boll shell volume and 100-seed weight from 10 to 40 days after flowering (DAF), and insecticidal protein contents in boll shell and cotton seed from 10 to 50 DAF. As expected, the differences between cultivars were significant for all the four characteristics. Cultivars with bigger boll shell volume had higher 100-seed weight but lower insecticidal protein contents in both of the boll shell and cotton seed. The correlation between boll size and boll insecticidal protein content was negative (−0.653). The correlation between 100-seed weight and cotton seed insecticidal protein content was significantly negative (−0.645). In 2005, leaf cut (LC) and square removal (SR) treatments were applied to Sikang 1 and Sikang 3. The boll shell volume and 100-seed weight were decreased by LR, but increased by SR. LC significantly enhanced and SR significantly reduced the insecticidal protein contents of boll shell and cotton seed. The correlation between boll size and boll insecticidal protein content (−0.870) and the correlation between 100-seed weight and cotton seed insecticidal protein content were both highly significant and negative (−0.841). Therefore, the results of the study indicated that the boll insecticidal protein content was significantly affected by boll size. The implications of the observed results in breeding, cultural practices and pest management were discussed. [Copyright &y& Elsevier]