The effect of exotic tree species on the flora of their undergrowth was investigated in an afforested site of Sèmè in Benin. The main vegetation in the area consists of timber tree plantation and fallow. Widely distributed species were in the majority (68.8% of spectrum) compared to guineo-congolian species (16.9% of spectrum). These plantations were poor in indigenous species and housed an undergrowth quite different from the native flora surrounding the plantations. This impoverishment of the original floristic diversity was related to ecological effects (shallows, litter amount, soil acidity, competition, allelopathy, etc.) induced by exotic trees (Acacia auriculiformis, Acacia mangium, Casuarina equisetifolia and Eucalyptus camaldulensis). In the Acacia plantation, undergrowth was covered by Acacia pods, stems, phyllodes and a litter amount of 21.05 t MS/ha. The Casuarina equisetifolia plantations presented also a high amount of litter and low plant species richness in the undergrowth. Eucalyptus camaldulensis plantations negatively influenced flora diversity through soil acidification and allelopathy. Casuarina equisetifolia alone or in association with Acacia auriculiformis increased the floristic diversity of the undergrowth.