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Growth stage and seasonal influences on the chemical composition and in vitrodigestibility of napier grass (Pennisetum purpureum) were studied in the sub-tropical desert climate of Peru's central coast. Important seasonal differences in the effects of plant maturity were noted. Dry matter concentration, steadily increasing with maturity, was generally higher in summer and lower in winter, compared with other seasons. The expected decline in crude protein with maturity was more rapid in the warmer seasons than in winter. Likewise, the levels of total cell-wall constituents (CWC), acid-detergent fiber, and cellulose increased more rapidly in the warmer seasons than in winter. The CWC fraction was less digestible in winter than in spring or fall, but in vitrotrue digestibility of dry matter (DMD) was lower in summer than in other seasons at comparable ages of the forage. Using CWC and lignin levels to predict %DMD resulted in a coefficient of determination equal to 0.79; holocellulose or lignin plus silica fractions were less reliable predictors of digestibility.The results demonstrate the desirability of early utilization of napier grass grown under Peruvian coastal conditions, at no later than 12 weeks of age in summer but at possibly slightly longer intervals in winter. Excess summer forage possibly could be carried over in the field for winter use, but winter and spring forage should be harvested before the onset of summer.