During the day the sand burrowing mysid Gastrosaccus psammodytes is confined to the intertidal and surf zone. There is a clear pattern of intraspecific zonation with adult males, immatures and juveniles furthest offshore. Females move progressively inshore as the brood develops, with young-bearing females occupying a zone closest inshore. At night G. psammodytes is present in the plankton when part of the population moves into deeper water and away from the area occupied during the day. Zonation appears to be related to the degree of planktonic activity. Loss or mortality of developing embryos is high in G. psammodytes and inshore movement of brooding females is interpreted as a movement away from excessive turbulence and sand movement in the area of breaking waves.