The epibenthic ophiuroid Ophiura sarsii forms widely distributed, dense beds in the upper bathyal zone around northern Japan. Population densities ranged from about 30 to several hundred ind. m-2. Size-frequency distributions were polymodal, and were usually dominated by a peak of large individuals which was probably composed of many year classes. Maximum size was found to increase with increasing depth in the area off Otsuchi, Pacific coast of northeastern Japan. Relatively strong recruitment was recognized in the shallower part of the dense bed, where O. sarsii predominated in the absence of other ophiuroid species. At greater depths, modes of small individuals were inconspicuous, and O. sarsii attained a larger maximum size. Comparisons of size structure at various localities showed that size was greatest at lower population densities, probably through density-dependent effects. Consequently, biomass was relatively constant at the different localities.