Abstract: The oak leafminer, Phyllonorycter messaniella, is a common defoliator in Europe where its damage usually remains occasional and limited. This insect has been observed for the first time in northwestern oak forests of Tunisia in 2012. The study, which was the first performed on this pest in Tunisia, aimed at acquiring a set of data on the pest bioecology in two zen oak forests, Mzara and Ain Zena. The rate of infestation of leaves with mines was high in Mzara (41.5%), whereas it was low in Ain Zena (17.6% of infested leaves). The number of collected larvae was higher in Mzara than in Ain Zena with 187 and 66 larvae, respectively. The average length of the collected pupae was 7.8 mm. The average duration of the pupal stage was 12 days. Characteristics of the developmental instars of larva, pupa and adult stages of the oak leafminer are also described.