Abstract: Acacia mearnsii (De Wild) is an Australian native leguminous tree, which has been introduced since 1970 into the cork oak forest of El Kala National Park (PNEK), north-eastern Algeria. Because of its development, this acacia is a threat to cork oak (Quercus suber L.).This work aims to study the effect of the A. mearnsii and Q. suber in mono- and co-culture conditions in soils collected from non-invaded sites, recently invaded or completely invaded and analyze its effect on the Quercus growth and arbuscular mycorrhizal (AM) of the two plants in controlled conditions. The results show a negative effect of acacia on cork oak growth, depending on the soil type, but no difference according to the conditions of mono- or co-cultivation. Acacia plants grown in soil from the totally invaded site are more colonized by arbuscular mycorrhizal fungi than those of the two other studied stands. Concerning cork oak, root mycorrhizal colonization rates by AM fungi is very low since this species forms predominantly ectomycorrhizal associations.