ABSTRACT.    The two-host life cycle of the Myxosporea (myxospores in a vertebrate, usually a fish and actinospores in an invertebrate host typically oligochate worm) is provided by many experimental cases of cross infestations which are carried out to date. How ever the differences between the Actinospores and Myxospores of the same species are such that their association is often impossible. The Myxospores have a non-planktonic structure, and the majority infects a vertebrate host in relation to the aquatic environment (typically a fish). The Actinospores result from the sexual reproduction in the oligochate worm and their dissemination is usually planktonic. The form Actinospore is the result of the sexual reproduction of Myxozoa. It takes place in an enclosure call pansporocyste made of two or four cells. This last contains two diploids cells which are responsible of the meiosis fallowed by an additional division. Eight gametes of each type alpha and beta are formed. The union of these gametes will give eight zygotes which will always be contained in the pansporocyste. Each zygote will develop in the actinospore with planktonic capacity. In some cases each of these spores will continue its progress individually, separate of the others. But sometimes (in some groups), all of the eight spores, are formed at the same time in the pansporocyste, will remain united by their external cells (epi-sporale), transformed into floats. Apparently this structure increases their planktonic capacity. These associations give the remarkable architectures used in theirs classification. A new case is presented. The eight spores resulting from a pansporocyste are joined together by a valvar cell transformed into float and form a star with eight heads. This structure is unknown until now. Those characteristics make it possible to attach it to the group Synactinomyxon (Stolc, 1899). It's a new form named Synactinomyxon aydatensis in relation with the Lake Aydat (Puy de Dôme, France), which is the place of its discovery, and the host is Eiseniella tetraedra (Annelid Oligochaete). One out of eighteen E. tetraedra is parasited by this Actinospore.