The peach fruit fly, Bactrocera zonata (Saunders) (Diptera: Tephritidae), is a serious pest in the last decade attacking a wide range of fruits in Egypt.  The esterase isozyme patterns during the three different stages (larvae, pupae and adults (males and females) of three wild populations (Alexandria, El-Fayoum and Siwa) and laboratory strain were observed in agar-starchpolyvinylpyrolidine (P.V.P) gel.  Two esterase isozyme (Est.a1and Est.a2) in anodal and three bands in cathodal (Est.c1, Est.c2 and Est.c3) for all stages were found.  The esterase activity of female was more than that of male in the three tested wild populations and laboratory strain.  The population of Siwa had greater esterase activity than those of El-Fayume and Alexandria.  Generally, the wild populations were found to have more esterase activity than the laboratory strain.