A comparative survey of both benthic and pelagic populations of the cumacean Bodotria similis was conducted in Oura Bay, Izu Peninsula, Japan. In the benthic populations, Bodotria similis, Bodotria pulex, Cumella spp., and Dimorphostylis spp. were dominant, and B. similis and B. pulex were spatially segregated throughout the year. The density of B. similis in the benthic population was low in summer and autumn, increased during winter, and was highest in early spring. Seasonal fluctuations in size classes and sex ratios within the benthic population were minimal, and ovigerous females and manca larvae occurred throughout the year. Size-frequency histograms of the benthic population suggest the presence of six developmental stages for each sex and that females produce more than one brood in their lifetime. Nighttime patterns of occurrence of planktonic Bodotria similis varied seasonally but peaked just after the sunset in spring and winter. The swimming individuals consisted of males and young, but ovigerous females were not found. Many manca larvae and juveniles with soft carapaces appeared from November to March, and large males with hard exoskeletons dominated in April and May. Nocturnal swimming of Bodotria similis is probably related to the molting of young, range extension, and possibly the mating behavior of adults.