This is a report of the qualitative and quantitative study of diatoms found in bottom-deposits of the Mutsu Bay, the northernmost bay of Honsyu, Japan., Samples were collected at 25 stations in the bay (fig., 1) in May, 1953., Table 2 and fig., 2 show the number of diatom-cells included in one gram of samples (dry) from each station., Table 2 suggests the following conclusions : (1) The factor which controls greatly the quantity of diatoms in the deposits is the resistance of cells to destruction by solution and current action., (2) Thala'nema, Melosira and Coscinodiscus are abundant in the deposit whereas Chaetoceros, Rhizosolenia, Skeletonema and Asterionella are almost entirely absent, though in plankon the latter very abundant in this bay., (3) The diatoms on the bottom of this bay are rather uniform in quality but not in quantity.,
