Variety resistance became important in beet breeding program strategies as an effective conventional alternative in pest control, where pests cause side effects of pollution and harmful impact on the plant, natural enemies and ambient environment. Whereas, variety resistance refer to the ability of some genotypes to give higher yields of good quality than susceptible ones at the same initial level of insect attack under similar environmental conditions. Therefore, this study was carried out under two geographical locations, Ismailia and Al-Fayoum Governorates in 2017/2018 and 2018/2019 seasons to determine the seasonal fluctuation in the population density of the sugar beet fly, Pegomyia mixta on eight sugar beet varieties (Florima, Gazelle, Tarbelli, Betamax, Sirona, Lilly, Hercule and Perikles) under natural infestation. Also, measuring the infestation effects on average root weight and sucrose percentage. The obtained results showed various noticeable signs of Pegomya mixta Vill. Infestation, expressed as population density (number of larvae), number of blotches, number of infested plants per 10 plants and infested leaf percentage were less in Ismailia area than those in Al-Fayoum, in both seasons. Further, Tarbelli and Lilly varieties exhibited the least resistance to beet fly infestation, which exhibited significant and the highest signs of infestation by beet fly, in both seasons and locations. On the other hand, Hercule and Sirona varieties recorded the highest resistance to beet fly infestation recording the lowest noticeable signs of infestation in both seasons and governorates. Regarding seasonal fluctuation of beet fly, the obtained results illustrated that the beginning time of beet fly infestation differed significantly among the used varieties. Moreover, some varieties were free from infestation till 15 and 18 December, in both seasons and locations. Meantime, for all of the tested varieties, the beginning time of beet fly appearance was in January in both seasons and locations. Data also manifested that the number of peaks of noticeable signs of beet fly infestation varied among the used beet varieties, in both seasons and locations. As for the consistent and obvious relationships between the coexistence of beet fly insect and the evaluated varieties, the results clarified that may be, there is no option for the insect to attack any variety. The obtained findings showed that whether in healthy or infested beets, root fresh weight and sucrose content were higher in Ismailia than those in Al-Fayoum. Data also cleared that the percent of reduction in average root weight and sucrose content resulting from infestation of plants by beet fly as compared to the healthy plants in Ismailia area were significantly less than those in Al-Fayoum area in both seasons. Meanwhile, the highest reduction of root weight and sucrose% were recorded by Tarbelli and Lilly varieties at Ismailia and Al-Fayoum Governorates. However, the lowest reduction in root weight was shown by Hercule variety, in Ismailia and Al-Fayoum, in the first season, and by Perikles and Sirona varieties, in the second one, due to the infestation by beet fly. Florima variety recorded the lowest reduction in sucrose content in both locations, in the first season. However, Sirona and Perikles varieties gave the lowest sucrose reduction, in the second one in both locations.