1. The ionic fluxes between the medium and haemolymph have been studied in adult Artemia salina, under steady-state conditions using 24Na and 82Br.
2. Extremely high fluxes have been found, the flux increasing markedly in the more concentrated media.
3. The results are interpreted as indicating that both the sodium and chloride in the haemolymph are undergoing rapid exchange diffusion with the sodium and chloride in the medium.