Alphitobius diaperinus, the lesser mealworm, is a key pest of poultry in broiler and layer facilities. The highly virulent INTA Mo4-4 strain of Bacillus thuringiensis is the most effective choice for controlling larval A. diaperinus. This study evaluated its sublethal effects on development, survival and fitness. Initially, six concentrations of INTA Mo4-4 spores and crystals were incorporated into the diet of A. diaperinus starting with 4-day-old larvae to assess virulence. Mortality was recorded after 14 d at 28 °C, and LC₃₀ and LC₅₀ were determined from four independent bioassays, yielding values of 69 and 136 µg/ml, respectively. Subsequently, sublethal bioassays using the LC₃₀ and LC₅₀, alongside a control group, determined the effects on surviving larvae. After 14 d at 28 °C, 791 larvae from five independent bioassays were analyzed. Their weights and larval areas were measured and later monitored to determine the durations of their larval and pupal stages, as well as their pupal and adult weights and body areas. Sublethal effects were clear in the LC₃₀ and LC₅₀ treatments, showing significantly reduced weights/areas per larva (0.25 mg/0.97 mm² and 0.20 mg/0.82 mm², respectively) compared to controls (0.37 mg/1.32 mm²). Although these trends persisted, differences were no longer statistically significant for developmental periods, pupae or adult fitness traits. Average larval (76.7, 83.1 and 104.6 days) and pupal periods (5.7, 6.4 and 7.0 days) and pupal (10.08 mg/9.32 mm², 8.75 mg/8.44 mm² and 8.12 mg/7.80 mm²) and adult weights/areas (8.97 mg/9.63 mm²; 7.65 mg/8.54 mm²; and 6.96 mg/7.95 mm²) per specimen followed similar trends across the control, LC₃₀, and LC₅₀ treatments, respectively. This study showed that INTA Mo4-4 induces both mortality and sublethal effects that could affect the development and fitness of A. diaperinus, offering valuable insights for bioinsecticide development targeting this pest.