Performance of eight Bt and non Bt cotton genotypes for American boilworm, Helicoverpa armigera (Hubner) infestation and its larval population was evaluated at Research Farm of CCS Haryana Agricultural University, Hisar. The infestation due to American boliworm and its larval population was nil in Bt cotton genotypes both under unprotected and protected conditions. The infestation in non Bt genotypes under unprotected conditions was maximum in RCH 134 (2.39%) and minimum in RCH 317 (0.88%), while under protected conditions it was maximum in ANKUR 651 (0.67%) and minimum in RCH 317 (0.10%). The larval population in non Bt genotypes was recorded maximum in ANKUR 2534 (1.24) and minimum in RCH 134 (0.52) under unprotected conditions. However, under protected conditions, the larval population was recorded maximum in RCH 138 (0.89) and minimum in ANKUR 651 (0.33). The maximum yield in Bt genotype was recorded in RCH 134 (27.84 and 30.04 q/ha) under unprotected and protected conditions, respectively. While in non Bt genotypes, maximum yield in ANKUR 2226 (12.75 q/ha) under unprotected conditions and in RCH 134 (25.37 q/ha) under protected conditions was recorded.