The present work was conducted during two successive seasons 2016 / 2017 and 2017/2018 in Diarb – Nigm district, Sharkia Governorate, Egypt to study assessment the damage of yield of sugar beet to infestation of Pegomyia mixta (Vill). The results showed that, the relationship between the yield of sugar beet and infestation of P. mixta was negative correlation i.e.; the increasing larval infestation caused the decreasing yield vice versa. The economic threshold level for P. mixta was 3.7 larvae /plant and the economic injury level was 4.1 larvae / plant in the first season, while, in the second season, the economic threshold level for P. mixta was 3.9 larvae / plant and  while, in the second season the economic injury level was 4.0 larvae / plant.