The laboratory evaluation of certain released Indian transgenic Bt cotton hybrids (RCH 2 Bt , RCH 515 Bt , RCH 596 Bt, RCH 530 Bt, RCH 134 Bt and RCH 533 Bt) for their efficacy on Spodoptera litura (Fab.) fourth and fifth instar larvae was done and comparison was made with non Bt cotton (non Bt cotton viz., Non Bt bunny and RCH 2 non Bt cotton) plant parts, young green bolls, top fully opened young leaves, middle leaves and squares. Studies were conducted at Insectary, Department of Agricultural Entomology, Tamil Nadu Agricultural University, Coimbatore during 2009 -2010 to study the effect of transgenic Bt cotton hybrids viz., RCH 2 Bt young green bolls which showed highest per cent mortality (15.86 %), top fully opened young leaves (14.50%), squares (13.50%) and middle leaves (13.50 %) and observed at 168 hrs after the treatments (HAT) followed by green bolls, top fully opened young leaves, squares and middle leaves of RCH 515 Bt that recorded 12.00, 10.50, 10.00 and 9.50 per cent mortality respectively after 168 HAT when compared to green bolls, top fully opened leaves, squares and middle leaves of RCH 2 non Bt cotton which recorded no mortality at all after 168 HAT. All four plant parts of Bt and non Bt cotton (green bolls, top fully opened young leaves, squares and middle leaves) recorded no mortality in fifth instar larvae of S. litura after at 168 HAT.