We assessed seasonal variation in food habits of Indian giant flying squirrels (Petaurista philippensis) and food availability at Fushan Experimental Forest in Taiwan. Between July 1997 and December 1998, giant flying squirrels consumed at least 79 species-specific parts belonging to 30 plant species from 19 families. Castanopsis cuspidata was eaten most often (24.95% of annual diet), followed by Ficus superba (14.67%) and Glochidion acuminatum (12.18%). P. philippensis fed mainly on young leaves (27.7% of annual diet) and mature leaves (24.0%). Leaf parts (including buds, petioles, young leaves, and mature leaves) constituted 74.0% of the annual diet. Monthly and annual diet variation was obvious, and this was related to availability of species-specific plant parts. P. philippensis fed on mature leaves and petioles in winter; buds, young leaves, and fruit in spring; and young leaves and fruit in summer and autumn. They preferred buds, young leaves, flowers, and fruit to mature leaves and petioles. However, mature leaves and petioles seemed to be relied on when more favorable food items were scarce.