A field experiment was conducted during kharif 2001-02 at the Agricultural Research Station, Dharwad Farm to assess the performance of three Bt transgenic hybrids under completely unprotected conditions. The experiment was laid out in a randomized block design with eight genotypes replicated three times. The genotypes were MECH-12 Bt, MECH-162 Bt, MECH-184 Bt, MECH-12 nonBt, MECH-162 non-Bt, MECH-184 non-Bt, DHH11 (local controlcontrol) and NHH-44 (national control). The results indicated that there was no effect of Bt-toxin on any of the sucking pests and that the presence of the cry1Ac protein in cotton hybrids created much variation in the incidence of bollworms. The population of Helicoverpa armigera Hüb. larvae was significantly lower in the Bt genotypes compared to the non-Bt and standard control hybrids. MECH-184 Bt registered a significantly lower incidence (0.91 larvae/plant), followed by MECH-162 Bt (1.05 larvae/plant). The population of spotted bollworms (Earias spp.) was also lower (0.06 to 0.08/plant) in Bt genotypes as against significantly higher (0.12 to 0.16 larvae/ plant) incidence in non-Bt genotypes. Similarly the incidence of pink bollworms was also lower in Bt genotypes compared to non-Bt genotypes as indicated by the population of PBW larvae in green bolls and the difference was significant. The effect of the Bt gene was impressive in terms of damage caused to fruiting bodies in different hybrids, among which MECH-184 Bt exhibited only 4.04 percent damage followed by 162 Bt (5.02%) and 12 Bt (6.84%) which were significantly superior to respective non-Bt versions and controls (15.89 to 21.30%). The locule damage was also less in Bt genotypes. Hence the yield of seed cotton was higher in MECH-184 Bt (12.13 q/ha) followed by 162 Bt (8.44 q/ha) and 12 Bt (6.77 q/ha). The yield in non-Bt hybrids was 8.12, 4.15 and 5.34 quintals of seed cotton per hectare from MECH184, 162 and 12 hybrids respectively in unprotected conditions. The studies clearly indicated the necessity of protection against sucking pests in Bt-cottons and possible reduction in number of insecticidal applications for management of boll-