<p>Pink bollworm strains (F₀) collected from 59 geographically different locations across India were advanced to F₁ and screened for <i>Cry</i> toxin resistance using 21-days diet incorporation bioassay. The survived population from five locations with the highest LC₅₀ values for <i>Cry</i>1Ac were grouped as highly resistant, while those from five locations with lower LC₅₀ values were grouped as relatively less resistant. Similarly, populations with high and relatively low resistance to <i>Cry</i>2Ab were also developed. Further, these populations advanced to next generation. One-day-old neonate larvae from all populations, along with a susceptible strain (F₁₉₄), were exposed to various selected doses to evaluate the precise impact of these concentrations on biological fitness. The results indicated that higher concentrations of <i>Cry</i> toxin prolonged the larval, pupal, and total developmental durations, while reducing adult longevity, fecundity, and egg hatchability. Egg incubation period, however, remained unaffected. Susceptible strains on toxin-free diet exhibited the shortest development times (larval: 18.48 ± 0.95 d; pupal: 7.22 ± 0.36 d; total: 39.41 ± 2.52 d), longest adult life (9.78 ± 0.35 d), highest fecundity (123.15 ± 4.95 eggs/female), and greatest survival rates (&gt; 95%). Their growth (5.28) and survival indices (0.97) reflected robust development under non-stress conditions. Conversely, toxin exposure increased mortality and malformations, particularly in susceptible strains. Fitness costs due to <i>Cry</i>1Ac exposure were highest in susceptible strains (84.42% at 1.0&#xa0;µg/ mL), followed by less resistant (79.46%) and highly resistant strains (60.07% at 10.0&#xa0;µg/ mL). For <i>Cry</i>2Ab, the relatively less resistant strain showed the highest fitness cost (74.51%), followed by susceptible (58.82%) and highly resistant strains. Overall, fitness costs were closely linked to reduced fecundity, egg hatchability, larval survival, adult emergence, and prolonged development.</p>