The present investigation was carried out to evaluate the efficiency of two biopreparations of Bacillus thuringensis (MYX104 and MYX833) on certain biological parameters of the potato tuber moth Phthorimaea operculella (Zeller) under laboratory conditions.The results cleared that the tested biopreparation had a delayed effect on the fitness components of the insect. In general, all the treatments achieved a significant shortened moth longevity of both sexes, prolongation of larval and pupal durations, severe reduction in the number of resulted pupae, great increase of the rate of malformed pupae, and sharp decrease of the rate of emerged moths according to the tested concentration of each biopreparation. Moreover, the consequently reared generations after parent's one treatment that indicated so lowered viability of developing individuals and decreased numbers of emerged unviable weak and sterile moths followed by final termination of the life cycle, which revealed as distinct failure of (F3) development due to the former treatment of (P) one with the lowest tested concentration of each of MYX104 & MYX833 (25 ppm). That failure could be attributed to the cumulative effect of induced recessive lethal genes in both influenced sexes along the extended period of the following generations post- treatment of parents one, causing apparent drastic effects, which appeared at the beginning of (F3). The results also proved that MYX104 was more effective than MYX833 on the investigated parameters of the fitness component of the potato tuber moth .