The genetically modified cotton expressing Cry toxins from the bacteria Bacillus thuringiensis (transgenic or Bt cotton) was introduced to Mexico in 1996, achieving a high degree of adoption by cotton producers, due to its high effectiveness against lepidopteran pests, which resulted in a reduction of the use of insecticides and a significant increase in crop yields. This study was conducted to determine and compare the populations and levels of damage of the main insect pests in commercial conventional (non-Bt cotton) and transgenic cotton (Bt cotton) fields in the Comarca Lagunera, Mexico during 2016, after 20 years of planting Bt cotton. In four fields of each type of cotton samplings were carried out from June 30 to August 27 to evaluate densities and damages of bollworm, Helicoverpa zea (Boddie); beet armyworm, Spodoptera exigua (Hubner); pink bollworm, Pectinophora gossypiella (Saunders); boll weevil, Anthonomus grandis (Boheman); conchuela, Chlorochroa ligata (Say); and sweetpotato whitefly, Bemisia tabaci (Gennadius). Bollworm and beet armyworm populations were significantly lower in Bt cotton than in conventional cotton, with reductions from 70-100% and 90-98%. Damage to squares and bolls by these two pests were lower in Bt cotton than in conventional cotton, with reductions from 82 to 84%. The pink bollworm and tobacco budworm were not detected in any type of cotton. Damage to cotton bolls by boll weevils and conchuelas were low and did not differ between types of cotton. Sweetpotato whitefly populations did not differ between types of cotton and increased at the end of the crop season. These results show that Bt cotton (Bollgard II) expressing Cry1Ac and Cry2Ab toxins continues to be effective against bollworm and beet armyworm; while the pink bollworm and tobacco budworm are practically eradicated from this region. The Bt cotton showed no effectiveness against sweetpotato whitefly, boll weevil, and conchuela, as was to be expected because they are non-target pests.