A field experiment was laid out in a randomized complete block design with three replications on vertisol at Main Agricultural Research Station (MARS), University of Agricultural Sciences (UAS), Dharwad during the rainfed seasons of 2007 and 2008 to assess the performance of Bt and non-Bt cotton genotypes under leaf reddening situation. The experiment consisted of ten treatments (cotton hybrids) viz., T1-Bunny (BG I) Bt, T2-Bunny non-Bt, T3-RCH-368 (BG I) Bt, T4-RCH-368 non-Bt, T5-JK Durga (BG I) Bt, T6-JK Durga non-Bt, T7-Neeraja (BG II) Bt, T8-Neeraja non-Bt, T9-DCH-32 non-Bt hybrid and T10-Sahana non-Bt variety were sown at 90 cm x 60 cm spacing. Results indicated that, Neeraja (BG II) Bt produced significantly higher seed cotton yield (2483 kg/ha) as compared to the other genotypes. The yield levels of Bt and non-Bt cotton genotypes under study were in the range between 2483 kg/ha and 1131 kg/ha. The lowest seed cotton yield was observed in RCH-368 non-Bt (1131 kg/ha). Mean red leaf index at 90 DAS was 1.32 and it increased to 1.77 at 120 DAS among the genotypes. At 120 DAS, check Sahana non-Bt variety recorded significantly higher red leaf index (2.12) and it was at par with RCH-368 (BG I) Bt (1.97) and JK Durga (BG I) Bt (1.92). Between Bt and non-Bt genotypes, Bt genotypes recorded significantly higher red leaf index(1.82) as compared to non-Bt genotypes (1.63). Amongst Bt genotypes, Neeraja (BG II) Bt recorded significantly lower red leaf index(1.58) than others.