Since the introduction of genetically modified cotton (GM or Bt cotton) to Mexico in 1996, less insecticide is used, and cotton yields have increased significantly. Considering the controversy over potential environmental impact of Bt cotton, and particularly the lack of information on its effects on diversity of entomofauna, the objective of this study was to determine insect diversity in non-Bt and Bt cottons in the Comarca Lagunera, Mexico. Insects collected in both types of cotton were in the orders Coleoptera, Diptera, Hemiptera, Hymenoptera, Lepidoptera, Neuroptera, and Thysanoptera. Insects in the order Hemiptera were most abundant, followed by Diptera and Thysanoptera. Insects belonged to 40 families, with most abundant being Aleyrodidae, Anthocoridae, Thripidae, Noctuidae, Chrysopidae, Agromyzidae, Syrphidae, Coccinellidae, Aphididae, and Lygaeidae. In total, 35 species of insects were identified, of which 18 and 17 were entomophagous and phytophagous, respectively. The most abundant species were sweetpotato whitefly, Bemisia tabaci (Gennadius); minute pirate bug, Orius tristicolor (White); onion thrips, Thrips tabaci Lindeman; western flower thrips, Frankliniella occidentalis (Pergande); beet armyworm, Spodoptera exigua (Hübner); pirate bug, Orius laevigatus (Fieber); serpentine leafminer, Liriomyza trifolii (Burgess); Allograpta obliqua; convergent lady beetle, Hippodamia convergens Guérin-Méneville; and green lacewing, Chrysoperla carnea (Stephens). The total numbers of entomophagous and phytophagous insects were similar in both types of cotton. Only bollworm, Helicoverpa zea (Boddie), and beet armyworm were more abundant in non-Bt than in Bt cotton. The 33 other species were similar in abundance in both types of cotton. Based on Margalef, Simpson and Shannon-Wiener indices, insect diversity did not different between Bt and non-Bt cottons. Minute pirate bug, O. laevigatus, and to some extent insidious flower bug, Orius insidiosus (Say), might be species to use for risk assessment and management of Bt cotton in the region, because of their abundance and presence during most of the cotton-growing season and in adjacent crops.