Floral sex allocation at the individual and first-order branch levels and the relation between these levels were examined in Betula platyphylla var. japonica, a wind-pollinated monoecious tree. Floral sex allocation at the individual level varied with resource availability in a pattern similar to that predicted by the Masaka and Takada model (Journal of Theoretical Biology 240: 114-125). Thus, individual trees with few reproductive resources produced only female or male inflorescences, whereas individuals with many resources rarely had a high male ratio (i.e., number of male inflorescences/total number of inflorescences). Furthermore, the number of male inflorescences tended to reach an upper limit, whereas the number of female inflorescences increased monotonically with increasing reproductive investment. The patterns of floral sex allocation at the first-order branch level were analogous to those at the individual level. Thus, each first-order branch of B. platyphylla var. japonica behaves like an individual, and the floral sex allocation of a given branch does not necessarily represent the individual tree. The effect of the individual-level floral sex ratio on branch-level floral sex allocation indicates that branch behavior is controlled by the individual.