Fucoxanthin is an oxygenated carotenoid present in edible brown sea algae, and dietary fucoxanthin is recognized to exhibit various beneficial effects. In the present study, the effects of fucoxanthin on osteoclastogenesis were investigated using cells from the macrophage cell line RAW264.7, which have the capacity to differentiate into osteoclast-like cells when stimulated by receptor activator of nuclear factor kappaB ligand. Fucoxanthin significantly suppressed the differentiation of RAW264.7 cells at 2.5 microM, which was not toxic to RAW264.7 cells. Treatment with 2.5 microM fucoxanthin also induced apoptosis accompanied by activation of caspase-3 in osteoclast-like cells. On the other hand, 2.5 microM fucoxanthin did not decrease cell viability in cells of the osteoblast-like cell line MC3T3-E1, indicating that the apoptosis-inducing activity of fucoxanthin in osteoclasts is stronger than that in osteoblasts. The present in vitro study suggests that fucoxanthin suppresses osteoclastogenesis via the inhibition of osteoclast differentiation and the induction of apoptosis in osteoclasts, but not bone formation.