Actilasioptera gagné (Diptera: Cecidomyiidae) is a gall inducer associated with the gray mangrove, Avicennia marina (Acanthaceae) which hasn't been previously recorded in Egypt. Mangrove forests are unique functional ecosystems having much social, ecological, economic and biological importance. This work aimed to study the relationship between the gall marker Actilasioptera Gagné and the gray mangrove, Avicennia marina,in addition to evaluating the quantifications of phytochemical and proximate compositions of Avicennia marina leaves with galls and without galls, as well as their antinutrients compositions on the Red Sea coast. The results of phytochemical compositions showed that total flavonoids, flavonols, steroids, saponins, tannins, alkaloids, and total phenolic contents were statistically significant at p < /del>P< 0.05 and higher in Avicennia marina leaves with galls than without galls. The total flavonoids, flavonols, and phenolic acids in the Avicennia marina leaves with galls and leaves without galls were 181.93± 1.36 mg RTE/ g DW, 51.10± 0.56 mg RTE/ g DW, 20.27± 0.65 mg RTE/ g DW, 8.77± 0.51 mg RTE/ g DW, 70.42± 0.61 mg GAE / g DW, and 26.90± 1.25 mg GAE / g DW, respectively. In addition, total steroids, saponins, tannins, and alkaloids were significantly higher in the Avicennia marina leaves with galls than in leaves without galls. The high values of proximate contents were found in Avicennia marina leaves with galls than Avicennia marina leaves without galls. Furthermore, the highest levels of anti-nutrients were found in Avicennia marina leaves with galls: oxalates, phytates, and cyanogenic glycosides were 21.60± 0.52 mg / 100 g, 1.102± 0.08 mg / 100 g, and 6.98± 0.22 mg / 100 g, respectively. Actilasioptera Gagné can infest and damage mangrove plant leaf,  leading to the death of the plant leaf as a result of access of plant defense chemicals against insect pests.