The life cycle of the species forming the genus Marteilia (Phylum Paramyxea), which are marine protozoan parasites of molluscs, is still poorly known, although there is evidence of the need for intermediate hosts. To date, only the calanoid copepod Acartia (Paracartia) grani has been demonstrated to be involved in the life cycle of the parasite infecting flat oysters growing in ponds with a low biodiversity. However, no complex natural environments, such as bays or estuaries, have been studied for a similar purpose. Therefore, a survey for the presence of the protozoan Marteilia was conducted by PCR on the zooplankton community of a natural ecosystem, the Alfacs and Fangar bays in the Delta de l'Ebre (NW Mediterranean). Identification of zooplankton species found infected allowed two new Marteilia hosts to be proposed: the cyclopoida Oithona sp. and an indeterminate harpaticoida species. This is the first report of Marteilia in zooplankton species sampled from a natural enzootic area. (C) 2007 Elsevier B.V. All rights reserved.