ABSTRACT: Feeding rhythm, in terms of diel change in frequency of occurrence of empty stomachs and in gut pigment content, and composition of diets were investigated for the sand-burrowing mysids Archaeomysis kokuboi and A. japonica during the summerseason on a sandy beach at Otsuchi Bay, northeastern Japan. A. kokuboi exhibits pronounced nocturnal feeding regardless of sex or developmental stage. A. japonica also shows nocturnal feeding, although the nocturnal feeding rhythm of themature mysids is less clear than that of the juveniles due to the increase in daytime feeding with growth. The nocturnal feeding of both species suggests that their previously known inter- and intraspecific habitat zonations during the daytime are notrelated to their feeding strategies for resources partitioning, but to other factors such as predator avoidance. Mature A. kokuboi mainly ingested planktonic copepods such as Microsetella rosea and Oncaea venusta, while mature A.japonica fed on benthic harpacticoid copepods as well as planktonic copepods. No difference between sexes was recognized in the diets of the 2 species. Juveniles of both species mainly fed on copepod nauplii. There was a maximum of 80% overlap in thediets of the juveniles of these 2 species. This overlap of diets explains the habitat segregation of juveniles of both species at night in the water column.