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AbstractIn 1996 transgenic Bt cotton was first grown on a commercial level in Arizona and the U.S. cottonbelt. Insecticidal properties of Bt varieties had been evaluated rather thoroughly in both the private and public sectors prior to commercial release. However, the agronomic characteristics had not been evaluated to any sufficient extent beyond the level of the developing companies. Lab and field tests were conducted in Arizona in 1996 dealing with the Delta and Pine Land Co. (DPL) companion varieties 5415/NuCOTN 33b (similar to 5415 but with the Bt gene) and 5690/NuCOTN 35b (with Bt gene). Most field comparisons were between 5415 and 33b. Lab and field studies revealed very similar agronomic characteristics between the companion varieties. No differences were detected with respect to heat tolerance, as determined by comparative fruit loss and abortion rates at the onset of the monsoon season. Only slightly higher vigor or growth rates were noted for 33b over 5415, which was considered to be negligible. Yield results revealed higher lint yields for 33b over 5415 in most cases. The difference in yields were attributed to pink bollworm infestations and damage, even when chemical control measures were being taken. It was concluded that 33b, as a transgenic version of 5415, is indeed very close to it's non-Bt counterpart.