The aims of this study were to determine the adaptability of giant king grass (Pennisetum hybridum) under Mediterranean climate conditions, and also to determine whether proper plant densities could improve growth and yield. Study site was located at Bornova, Turkey (38 degrees 27.236 N, 27 degrees 13.576 E and 20 m). Treatment consisted of four densities of plant population (D-1:57,143; D-2:28,571; D-3:19,048 and D-4:14,286 plant ha(-1)) with three replicates per treatment in a randomized block design. Setts were planted in mid June of 2010 and allowed to grow for 4 full growing seasons. Average result of four years indicated that there were significant effects of plant densities on the dry matter yield and other yield characteristics of giant king grass but not on silage pH. It was recommended that the production of giant king grass using D-2 (70x50 cm) was the most successful planting density regarding the dry matter (32.6 t ha(-1)) and crude protein yield to the regions with Mediterranean-type climates under irrigation.