Differences in susceptibility to insecticides based on adult insect sex indicate that males are more susceptible than females. Hence, it may be necessary to sex the insects before a bioassay is conducted, but this activity is tedious, takes time, and this manipulation might cause harm to the insects. Monitoring the evolution of resistance to the neonicotinoid insecticides in field whitefly populations is important and usually based on adult exposure to these toxicants. The aim of this study was to correlate the susceptibility to thiamethoxam with sex in susceptible populations of Bemisia tabaci (Genn.) B-Biotype and Trialeurodes vaporariorum (West.). Five-day-old males, females, and a mixture of female and male (80:20) adults were evaluated to determine log doses-Probit lines for this insecticide. In B. tabaci, the CL50 ranged of 1.4 mg [IA] L-1 to 2.8 mg and the relative proportion RR50 ranged from 1.0 to 2x. The CL95 ranged from 12.3 to 16.7 mg and the RR95 values varied from 0.8 to 1.0x. In T. vaporariorum, the CL50 fluctuated from 14.2 to 30.3 mg and the value of RR50 between 1.0 and 2.1x. The CL95 ranged of 180.4 to 314.0 mg and the RR95 from 1.0 to 1.7x. In both species, no differences were found between insect sex and susceptibility to thiemathoxam. Hence, to conduct bioassays with thiamethoxam on these species, the use of one sex or the other is irrelevant.