The population density of the aphid Melanaphis donacis (Passerini) was studied in experimental plots of Arundo donax (L.) at the Antumapu Campus of the University of Chile, La Pintana (33° 34' 08'' S 70° 38' 40'' W) and on wild populations at Rinconada de Maipú (33° 30' 0.17" S and 70° 49' 12.25" W), both in the Mediterranean zone of Chile. Sampling started upon the first colonization of the plants by the aphid on December 14, 2012, and continued every 15 days through December 14, 2013. Samples were taken from the 3rd leaf from the apex, and a total of 36 samples were collected per sampling date. The aphid occurred throughout the growing season of A. donax, especially in summer, with higher densities in the experimental plots than in the wild populations and means of 243 and 147 aphids leaf-1, respectively. The main natural enemies observed were the coccinellids Eriopis connexa (Germar) and Hippodamia convergens (Guérin-Méneville) and the syrphid Allograpta pulchra (Shannon). The predator populations followed the growth curves of the aphid population and of A. donax.