Bull. Jpn. Soc. Fish. Oceanogr. 75(1), Page 19-28, 2011   Diel changes in vertical distributions of two mysid species, Neomysis, in estuarine brackish water Satoshi Katayama1†, Motoya Tamaki2 and Akihiro Okata3 1 Coastal Fisheries and Aquaculture Division, National Research Institute of Fisheries Science, 6-31-1 Nagai, Yokosuka, Kanagawa 238-0316, Japan 2 Coastal Fisheries and Aquaculture Division, Seikai National Fisheries Research Institute, 1551-8 Taira-machi, Nagasaki, Nagasaki 851-2213, Japan 3 Laboratory of Hydrospheric Ecology, 1-2-6 Ushirono, Hitachinaka, Ibaragi 312-0014, Japan  † skata@affrc.go.jp In order to reveal vertical distribution patterns of two mysid species, Neomysis, we collected the mysids from surface, middle and bottom layers in Hiroura Lagoon located in the estuary of Natori River, twice in spring and neap tides, 1994. N. awatschensis and N. japonica coexisted in the lagoon. Although both species distributed mainly in the bottom layer, they occurred in the surface and middle layers in night time. Through correlation analysis, negative effect of illuminance on the density of mysids was found. Their distributions would be expanded to the surface due to their swimming activities enhanced in night. Relationship of stomach fullness and maturational condition with the diel vertical distribution of N. japonica was not significant; however, juvenile N. awatschensis and the mysid without full stomach contents tended to distribute in the middle and surface layers. N. japonica often disappeared from the bottom layer, while N. awatschensis stayed there almost all the time. Behavioral differences were shown between these species. Key words: Neomysis awatschensis, Neomysis japonica, vertical distribution pattern, maturational stage, gut fullness