Codakia orbicularisis a tropical lucinid harboring gill endosymbionts which are environmentally transmitted from a free living-symbiont form to the new host generation after metamorphosis. Structural changes occurring in the cellular organization from incomplete gill filaments in young aposymbiotic juveniles to full differentiated gill filaments containing bacterial endosymbionts in reared symbiotic juveniles, were analyzed for juveniles from 250 μm to 2 μm shell-length. Aposymbiotic juveniles possess differentiated gill filaments with ciliated, intermediary, and lateral zones similar to those described in wild juveniles, except for the bacteriocytes which are lacking. Granule cells, which progressively differentiate during the morphogenesis of the gill filament, do not appear as a consequence of symbiosis. Experimental colonization of aposymbiotic juveniles by the free-living symbiont form has been obtained through the addition of unsterilized sand collected from the natural habitat ofC. orbicularis.Two days after exposure to crude sand, symbiosis-competent bacteria enter by endocytosis at the apical pole of undifferentiated cells which progressively differentiate into classical bacteriocytes similar to those found in the adult gill filaments. Undifferentiated cells of aposymbiotic gill filaments remain receptive to bacteria several months after metamorphosis, and become bacteriocytes when aposymbiotic juveniles get contact with the symbiont free-living form. Therefore, the environmental transmission of symbionts does not appear to be restrained to a defined period of time during post-larval development inC. orbicularis.