Cryptocaryon irritans is a holotrichous marine ciliate protozoan that causes “white spot disease”, which isone of the most devastating parasites of Larimichthys crocea in Fujian Province. No safe drug strategy has been identifiedto prevent and cure “white spot disease”. To investigate the expression and prognostic value of macrophage migrationinhibitory factor (MIF) in C. irritans diseased L. crocea tissues, MIF expression was detected by immunohistochemistryof intestine, spleen, head kidney, and liver specimens of pathologically identified and healthy L. crocea tissues.None of the negative controls reacted, but MIF staining was intense in all L. crocea tissues infected with C. irritansdisease. The MIF staining rates for the intestines, liver, spleen, and head kidney were 54%, 80%, 86% and 90%,respectively. The high MIF expression levels in diseased tissues indicate that MIF plays an important role in homeostasisof C. irritans infected with L. crocea.