A field study evaluating 35 candidate commercial Bollgard ^® varieties at 11 different field sites showed that Cry1Ac levels (as measured in a sensitive quantitative bioassay) in various tissue types were influenced by field site, sampling time and variety. There was no measurable site by variety contribution to variability in Cry1Ac levels, suggesting that relative varietal differences were independent of site or environment. Based upon the reported data, it was suggested that differences in field efficacy for less sensitive species like fall armyworm and cotton bollworm, if they appear, are likely functions of differences in terminal/foliar levels of Cry1Ac as influenced by plant age, field site or varietal background. Fruiting structures (squares and bolls), the most attractive targets for attack by key pests, generally contained lower levels of Cry1Ac than did terminals. For fruiting structures, field site was a much larger contributor to overall variability than varietal background. For squares, specifically, plant age (tissue sampling time) was also a much larger contributor to Cry1Ac variability than varietal background. Significant differences in field efficacy due to varietal differences in fruiting structure Cry1Ac levels were considered unlikely. Current testing protocols require that all candidate Bollgard ^® varieties meet minimum Cry1Ac concentration standards in multi-site field studies as measured against square Cry1Ac levels found in Monsanto’s original EPA-approved 531 event in the Coker 312 background. Insect resistance management implications were discussed.