The digestibility of Napier grass (Pennisetum purpureum) in grass carp (Ctenopharyngodon idella) is estimated on the basis of the lignin content in the grass as an internal marker. The digestibility of Napier grass is 19.7% on dry matter basis. Chemical analytical data reveal that 65.54% protein, 53.62% lipid, 17.56% cellulose, 13.60% fiber and 25.34% ash were extracted from the feed. The data indicate that digestion of the grass relies mainly on the mechanical rupture of the grass cell wall during the grinding processes. Microflora do not seem to play a significant role in grass digestion or provide the fish with a source of bioprotein.