Examination of 1,323 American smelt, Osmerus mordax (Mitchill), from the Great Bay region of New Hampshire during the period from October 1951, through April 1952 showed that 308 (23.3 percent) were infected with Glugea hertwigi. The serious pathological manifestations of the disease suggested that it is an important contributing factor to the recent decline in the smelt population of the area. Although the mode of transmission of the parasite from smelt to smelt is not known, it seems probable that it enters with the food–perhaps after prior mechanical concentration of the spores by one or more of the organisms utilized by the smelt for food. Additional study is needed on the problem of transmission.