1. A fungus disease of epidemic proportions was found in the common sea herring (Clupea harengus) throughout the Gulf of Maine. 2. The fungus was also found to infect the common winter flounder (Pseudopleuronectes americanus) and the alewife (Pomobolus pseudoharengus). 3. The causative agent was found to be a species of fungus belonging to the genus Ichthyosporidium Caullery and Mesnil (1905). The specific name is tentatively accepted as hoferi Plehn and Muslow (1911). 4. The organism is believed to be a normal parasite to the herring and reaches epidemic proportions only when certain unknown factors are operative. 5. The causative organism was found in herring preserved in 1926, and it is believed that the epidemic has been increasing in severity since that time. 6. It is believed that such an epidermic, once initiated, increases in severity, reaches a peak, and subsides to a subpatent level. The peak is believed to have been reached in 1931. 7. The life history and effects of the organism in the herring and flounder are described. 8. The herring is believed to acquire the infection by the ingestion of parasites liberated from fish in the same school. 9. The flounder is believed to acquire the infection by the consumption of infected herring. 10. The alewife is believed to acquire the infection by ingestion of the parasite during its infrequent association with the herring. 11. Infection is believed to be established by way of the alimentary canal and, once established, to spread throughout the host by way of the blood stream or the lymphatics. 12. Direct cross infection from the herring to the flounder establishes the theory that the parasites in these two hosts are one and the same organism. 13. Direct cross infection experiments from the herring to the flounder eliminate the necessity of an intermediate host. 14. There is no reason to believe that this parasite is capable of infecting warm-blooded animals.