Epizootics of Elimeria funduli Duszynski, Solangi, and Overstreet, 1979, a pathogenic hepatic coccidian of killifishes (Fundulus grandis, F. heteroclitus, F. similis), have occured in many coastal areas of the northern Gulf of Mexico. All of more than 500 specimens of F. grandis examined from Mississippi Sound during 1977 to 1981 had infections, regardless of age, sex, or season collected. Oocysts of E. funduli primarily occur in the liver and pancreas, replacing up to 85% of both those organs. Other infrequent sites of infection include the ovary, intestine, and mesenteric fat. Eimeria funduli is the first known eimerian to require an intermediate host. To complete the life cycle, an infective stage in the grass shrimp Palaemonetes pugio has to be eaten by the killifish. Preliminary studies indicate that the coccidian affects the growth of killifish. Infected fish treated orally with Monensin® for 20 days exhibited a 50 to 70% reduction of oocysts as compared with untreated fish. Also, infected killifish maintained exclusively on a diet of TretraMin® for 3 months lost their infection.