Abstract Effects of elevated CO2 (twice ambient vs. ambient) and Bt Cry1Ac transgene (Bt cotton cv. 33B vs. its nontransgenic parental line cv. DP5415) on the interspecific competition between two ecologically similar species of cotton aphid Aphis gossypii and whitefly biotype-Q Bemisia tabaci were studied in open-top chambers. The results indicated that elevated CO2 and Bt cotton both affected the population abundances of A. gossypii and biotype-Q B. tabaci when introduced solely (i.e., without interspecific competition) or two species coexisted (i.e., with interspecific competition). Compared with ambient CO2, elevated CO2 increased the population abundances of A. gossypii and biotype-Q B. tabaci as fed on Bt and nontransgenic cotton on 45 (i.e., seedling stage) and 60 (i.e., flowering stage) days after planting (DAP), but only significantly enhanced aphid abundance without interspecific competition on the 45-DAP nontransgenic cotton and 60-DAP Bt cotton, and significantly increased whitefly abundance with interspecific competition on the 45-DAP Bt cotton and 60-DAP nontransgenic cotton. In addition, compared with nontransgenic cotton at elevated CO2, Bt cotton significantly reduced biotype-Q B. tabaci abundances without and with interspecific competition during seedling and flowering stage, while only significantly decreasing A. gossypii abundances without interspecific competition during the seedling stage. When the two insect species coexisted, the proportions of biotype-Q B. tabaci were significantly higher than those of A. gossypii on Bt and nontransgenic cotton at the same CO2 levels, and elevated CO2 only significantly increased the percentages of biotype-Q B. tabaci and significantly reduced the proportions of A. gossypii on seedling and flowering nontransgenic cotton. Therefore, the effects of elevated CO2 were favorable for biotype-Q B. tabaci to out-compete A. gossypii under the predicted global climate change.