AbstractEggshell peroxidase (ESP) is responsible for the hardening process in several Diptera, includingDrosophila melanogasterMeigen andBactrocera oleaeGmelin. Its action can be inhibited by phloroglucinol, a natural phenolic substance, during the formation of the egg-shell. We used phloroglucinol, in the diet of adults ofD. melanogasterandB. oleae, at concentrations of 1 mM, 10 mM, 25 mM, 50 mM, 100 mM and 400 mM to study its effect on egg laying. In both insects, 1 mM had no effect. At concentrations above 10 mM, egg laying ofD. melanogasterwas gradually affected leading to the deposition of empty shells and chorion-less eggs, while inB. oleae, it was gradually suppressed and only amorphous masses were laid. The effect of phloroglucinol at the tested concentrations was reversible. It is concluded that phloroglucinol added at appropriate, non-lethal concentrations, affected egg-laying ofD. melanogasterandB. oleaein different ways, related to differences in the ovipositor diameter relative to egg size. InB. oleae, it is relatively narrow, causing breakage of the phloroglucinol-induced non-elastic egg-shell, since covalent crosslinking of the chorion proteins is prevented. InD. melanogaster, chorion-less eggs and separate chorions are laid, due to egg dechorionation. A possible field application of these results is discussed.