Abstract: Phyllonorycter messaniella is a leafminer that infests numerous plants, mainly on deciduous oaks. In Tunisia, this leafminer was observed for the first time in 2013 on the kermes oak, Quercus coccifera, in north-eastern Tunisia. The aim of this work is to study the pest infestation on the kermes oak and the insect behavior. The study was done in spring 2015 in 4 stations in Jebel Abderrahmane. Host-plant infestation was evaluated by counting the number of infested leaves on 30 kermes oak trees in each station. Large and small diameters of host-plants were measured to calculate the average diameter (Dm). Infested leaves were collected, scanned, and the leaf area was calculated using the Image J Launcher software. Beni Oulid was the most infested station with a mean of 5 infested leaves/tree, whereas Ftahiz was the least infested with 1.73 infested leaves/tree. In small leaves, infestation was observed on the entire leaf. In Beni Oulid, the average crown diameter (Dm) was 145.68 cm, on a leaf area of 5.19 cm2, the leafminer can burrow 1.09 cm2. In Delhiza, with Dm = 114.5 cm, 0.90 cm2 of 2.88 cm2 was defoliated. In Ftahiz, with Dm = 113.8 cm, 0.82 cm2 of 3.20 cm2 of the leaf area was defoliated. In Guitoun, Dm = 122.1 cm, 1.28 cm2 of 3.63 cm2 was defoliated. More studies on host plants of P. messaniella will be carried out.