A 12 field site study was performed in which DP50B (Bollgard^®)cotton tissue samples were collected throughout the growing season and evaluated for Cry1Ac concentrations utilizing a sensitive quantitative bioassay. Annual seasonal means for terminal foliage, pre-candle squares, and young bolls were 22, 14, and 17 g/g dry weight, respectively. A main-effect ANOVA determined that field site, sampling time, and plant tissue type were all significant sources of variability among Cry1Ac levels in DP50B. Mean Cry1Ac concentrations within specific tissues, although variable from sampling time to sampling time, showed no specific trend over time to either increase or decrease; this suggests that tissues of similar physiological age may express Cry1Ac at levels around a tissue-specific mean throughout the fruiting cycle (the approximate sampling period of this study). Further, this tissue-specific mean may be influenced by environmental conditions at specific sites.