A parasitological survey was carried out during 1994-1996 in Mazandaran and Guilan Provinces (Caspian Sea). 159 samples from 17 different species were examined. Samples were collected from Tajan River, Anzali Lagoon and some fish farms. Immediately after caught live fish were transported to laboratory and kept in aquarium for further examination. During this study ten species of Myxosporean parasites were separated, identified and classified. This is the first report on infestation of inland fishes of Iran by these ten parasite species. Morphology of spores, development stages of plasmodia, tissue specifications and cyst production of these parasite species were investigated in the present study. Four of the Myxosporeans were identified as potential pathological hazards for three fish species. These are Myxobolus pavlovski for Silver Carp (Hypophthalmichthys molitrix) and Big Head (Hypophthalmichthys nobilis), M. bramae for Kutum (Rutilus frisii kutum). M.muelleri and M. ellipsoides for Cyprinids. The other six identified Myxoporid species are: Myxobolus pseudodispar, M. minutus, M. musculi, M. musayevi, M. samgoricus and Myxidium pfeifferi