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We observed ostrich (Struthio camelus) in southern African savanna, desert grassland, arid shrubland, and fynbos (Mediterranean shrubland) to determine physical and chemical factors that influenced food selection. Natural diet information was needed to provide guidelines on carrying capacity of natural vegetation for ostrich and for improving rations for captive ostrich. Ostrich fed on green annual grasses and forbs when available. When these were not available they consumed leaves, flowers, and fruits from succulents and woody plants. Dead or woody material and animal matter (other than bone) were absent from the diet. Ostrich selected (P < 0.05) for fiber and against (P < 0.05) phenolics in forbs. They preferred (P < 0.1) protein in graminoids and fiber (P < 0.05) in succulents, but avoided (P < 0.05) calcium in succulents. They selected (P < 0.05) sodium and avoided (P < 0.05) ether-extractable substances in woody plants. Ostrich did not feed on plants toxic to mammalian herbivores. Ostrich need 5-6 kg fresh mass daily when feeding on natural forage containing 70% water (on a dry mass basis, 24% fiber, 12% crude protein, 16% ash, 3% lipid) for maintenance. Ostrich mobility and selectivity, which enables them to obtain adequate, high quality forage in arid environments, makes them destructive to rangeland when confined and stocked at high densities.