Freeze-fracture technique has been employed to investigate the supramolecular structure of the tegumental outer membrane of Schistosoma mansoni, and to check structural changes during the development from cercaria to adult worm. We can now distinguish at least two main categories of cercarial intramembranous particles (IMPs) which are seen on P fracture face (the outwardly directed fracture face of the inner leaflet). During the alteration of the cercarial body we have found a striking difference in the distribution of particles between the two fracture faces. The elongated partices which correspond to an alignment of four subunits go deeply into the cytoplasmic leaflet. At this time another class of isolated particles (ranging from 66 to 90 ) became prominent, whereas a significant increase in particle density could be seen on the E face. The particle density repartition in the four fracture faces obtained from 0- to 60-min schistosomula prepared from skin or artificailly reveals three kinds of observations. (i) Cercarial particles can be recognized on fracture faces of the inner membrane; (ii) one class (likely the second class described in “destabilization” conditions) leads to self-gathering into aggregates; (iii) in these areas, the intramembranous components show a variation in the partition coefficent. The formation of aggregates which coincides with the migration of glandular structure into the tegument may be the primary change involved in the formation of the new outer membrane. It is suggested that during the schistosomulum evolution some IMPs may migrate with or into the new membrane. The P 1, E 1, and P 2 faces of adult worm membrane are almost devoid of particles, whereas the E 2 face shows a random distribution of numerous and large IMPs. The structural and functional significance of those findings are discussed.