Benthic foraminifera are investigated in Aso-kai Lagoon, central Japan, to understand the habitat of the anoxic environment. The condition of the deep bottom is anoxic almost throughout the year. Five taxa are commonly found in this lagoon, of which Virgulinella fragilis dominates the inner to central area and the deeper parts of the lagoon. The occurrence of the other common taxa, such as Trochammina cf. japonica, Ammonia cf. A. beccarii forma l, Rosalina spp. and Elphidium sp.A, is restricted to the adjacent part of the sand bar, called Amano-hashidate. V. fragilis has a very high tolerance to almost year-round anoxic condition. The results suggest that the seasonal changes in the dissolved oxygen levels of the bottom waters mainly control the distribution of benthic foraminifera in this lagoon.