The effect of indole-3-acetic acid (IAA) on flowering in apple was investigated by using 'Fuji' (biennial bearing cultivar) and 'Ohrin' (regular-bearing cultivar) cultivars (Malus pumila Mill.). IAA concentration in the seed of 'Fuji' was higher than that of 'Ohrin' 28 days after full bloom (DAFB) to 108 DAFB. The IAA levels in 'Ohrin' peaked later than they did in 'Fuji'. After 55 DAFB, IAA levels in the bourse of fruiting trees were higher than those of trees which had undergone non-fruiting deflorated treatment. The IAA levels in the seed and bourse of 'Fuji' peaked at 77 DAFB which coincide with the time of flower bud initiation. These results suggest that IAA may be related to flower bud formation in apple.