Two experimental designs were carried out at Fayoum Governorate during mature fruit seasons of 2006 and 2007 to establish different levels of infestation on the economic injury level of the Mediterranean fruit-fly (Medfly) Ceratitis capitata (Wied.). In the first trial, four mandarin trees at different levels of infestation were surrounded by muslin cages constructed by wooden frames. Sticky traps baited with trimedlure were hung on each tree for evaluating adult fly density and the relationship between trap catch and infestation percent, total of living stages and crop loss. The second trial was carried out to establish a range of infestation levels across experimental field plots of mandarin using different insecticide treatments to evaluate the relationship between C. capitata density and the abovementioned parameters. High levels of C. capitata adults in trap catch were associated with significant reductions in fruit yield. The loss in mandarin yield was evaluated at different levels of infestation. High yield loss was obtained (903$/feddan) in mandarin plots when no control practices were applied.