Marteilia spp. are paramyxean parasites that affect several bivalve species of economic interest, such as Ostrea edulis and Mytilus galloprovincialis. Certain aspects of Marteilia spp., such as their life cycle and host affinity and infection dynamics, still remain unknown. The 'Delta de l'Ebre' constitutes a natural model for the study of the life cycle of the parasite Marteilia, since uninfected mussels and flat oysters immersed in the bays can become infected. This, along with the geographical and ecological characteristics of the bays, make it a very interesting location to study the Marteilia life cycle. Preliminary results concerning marteiliosis, mainly in mussels, such as prevalence dynamics, infectious periods, host affinity and host intermediate candidates are reported in the present paper. This information will be required for further, more exhaustive, studies in the bays of the Ebre delta.