Immunolocalization and ontogenetical changes in Na ,K -ATPase were investigated in the antennal glands, intestine, and branchial cavity in developing Homarus gammarus. The antennal glands lined by undifferentiated cells are detectable in embryos at stage EI 150 μm (EI: eye index; 640 μm at hatching). From a mesodermic sac and an ectodermic tubular epithelium (EI 225 μm), they develop into coelomosac, labyrinth, and bladder up to larval stage II. In larval stage III, the end parts of the labyrinth fold inward. In the first post-larvae, the coelomosac is completely enclosed in the folded labyrinth, there is no nephridial tubule. The definitive organization of the antennal gland found in adults is achieved in post-larval stage V. At EI 425 μm, the intestine epithelium is composed of cuboidal cells, and the branchial cavity is already formed at EI 325 μm. The presence of Na ,K -ATPase was detected on the basolateral side of ionocytes lining the epithelium sac of the antennal gland starting at EI 425 μm, in the intestine at EI 625 μm, and in the epipodites of the branchial cavity at EI 525 μm. In post-larvae (stage V), a strong fluorescence was also found on the inner-side of the branchiostegite epithelium. Thus the ontogeny of the osmoregulatory epithelia is completed only after metamorphosis and it conditions the occurrence of the adult pattern of osmoregulation.