ABSTRACT.    Paramecium is a polarized cell, with many specialized territories. Their distribution defines a global body plan which must be transmitted to the daughter cells. Epiplasmins are original proteins constitutive of part of the under-membrane skeleton: the epiplasm. The 51 sequences of this new multigenic family, the epiplasmins are characterized by a conserved central domain, flanked on both sides with specific structural domains. We show that the majority of the epiplasmins sets up a group of "symmetrical" type proteins, based on their ordered domain arrangement. This symmetry can be altered by the loss of C-terminal domains, which determines another protein type defined as "asymmetrical". A third type of proteins exists but does not show any particular geometry of domains. The functional study of the epiplasmins is carried out by using localization markers such as GFP and by altering gene expression by RNA interference (RNAi). Addressed proteins of both symmetrical and asymmetrical types show a uniform distribution in the cortical units of Paramecium while the others epiplasmins are addressed to their periphery. This differential localization suggests a particular role for each type of protein. Using RNAi of symmetrical or asymmetrical epiplasmins sequences leads to a similar phenotype. The cell cannot divide any more and loses its global polarity. This phenotype is not observed with the epiplasmins addressed to the periphery of the cortical units. In this case, under RNAi, a delay in cell division and an altered duplication of the new cortical units are observed. Thus, epiplasm and its components the epiplasmins represent a key structure that guide the success of the cortical units' multiplication, the specialized territories of the cell. Regarding the whole cell, it is also a structure which carries and transmits informations essential for the global body plan.