In order to evaluate the performance of transgenic cotton (Gossypium) varieties under Arizona conditions, 4 field tests were conducted at 3 Arizona locations in 1999 using a total of 34 varieties in one or more of the tests. Across locations and varieties, Bollgard (BG) [transgenic for Bacillus thuringiensis insecticidal endotoxins] and stacked (BG/RR) varieties produced about 7 to 8% greater lint yields than the conventional varieties. Roundup Ready (RR) [transgenic for glyphosate resistance] varieties produced similar lint yields to conventional varieties. A few transgenic varieties were lower yielding than the conventional parent in these tests. RR varieties tended to be taller and more vigorous than the conventional parent. Transgenic varieties were sometimes different from the conventional parent in other traits, such as lint percentage, boll weight or maturity, but the variation was not associated with a particular transgene.