The idea of thinning forest stands with a well established cover of understory vegetation has gained ground recently. In a 17 years-old Pinus pinaster plantation, established at a density of 1200 trees/ha in a semi-arid Mediterranean environment of Macedonia, Greece, three degrees of thinning (control, 50 per cent and 75 per cent of trees removed), two levels of fertilization (control and 1000 kg/ha of NPK fertilizer) were applied. All the stands were seeded with 30 kg/ha of Dactylis glomerata seed. Measurements included light intensity under trees canopy, density of Dactylis glomerata plants and herbage production. It was found that fertilization favoured the Dactylis glomerata growth and Dactylis glomerata was better established in low and in medium density stands with light intensities higher than 20 per cent.