The cuttlefish Sepia officinalis mate and spawn in the intertidal zone where eggs are exposed during low tide to osmotic stress. embryonic outer yolk sac is a putative site for osmoregulation of young S. officinalis embryos. By using electrophysiological recordings and immunostaining we showed, (i) that the chorion is only a passive barrier for ions, since large molecules could not pass through it, (ii) that a complex transepithelial potential difference occurs through the yolk epithelium, (iii) that ionocyte-like cells and Na + /K +-aTPases were localized in the yolk epithelium and (iv) that ouabain sensitive Na + /K +-aTPase activity could participate to this yolk polarization. These data warrant further study on the role of ion transport systems of this epithelium in the osmoregulation processes in S. officinalis embryos.