Dry matter productivities and growth parameters of napiergrass (Pennisetum purpureum Schumach, cv. Merkeron) were compared between the plants grown from seedlings with high fertilizer inputs at Tokyo and at Miyazaki. The dry matter yields of whole plants at the end of growing season were 39.3t/ha at Tokyo and 51.8t/ha in 1984 and 40.1t/ha in 1986 at Miyazaki. In comparison with Miyazaki, Tokyo had disadvantages for production such as shorter duration of productive period and lower levels of temperature as well as solar radiation, except for temperature level in summer, which was similar at the two sites. In summer, however, the rate of increase in leaf area index (LAI) at Tokyo was higher with a lower extinction coefficient and a higher net assimilation rate. This resulted in a steeper increase in crop growth rate (CGR) owing to the increase in LAI and a higher maximum CGR at Tokyo than at Miyazaki, and diminished the aforementioned disadvantages involved in the production at Tokyo.