The main aim of this study was to investigat e the clinical and postmortem finding, seasonal prevalence. And histopthological altertions that are caused by probably a new species of Myxobolus in ovaries of sharptooth catfish , Clarias gariepinus. In Assiut , Egypt Out of 120 fish examined over one year (2004) , ovaries of only 15 (12. 5%) fish were infested with macroscopic Myxobolus cysts (plasmodia and host cyst) that were embedded in the connective tissue among ova. Prevalence of infestation started low in late autumn and increased over winter and reached maximum in early spring . Infestation was not reco rded in summer. Six (40%) out of the infested fish had Myxoblus cysts in only one ovary , meanwhile, the reminder (60%) of infested fish had both ovaries infested . Also , intensity of in festation gradually increa sed over winter and was maximal in early spring . but abruptly declined in summer. Microscopic examination of plasmodia showed numerous typical Myxobolus spores at var ious developmental stages. Mature spores are oval in shape with two anteriorly located polar capsules that have 4-5 coils of polar filaments . Microscopic examination of infested ovaries reveale d that Myxobolus plasmodia were encapsulated within a thin connective tissue layer of host react ion. Myxoblous cysts compress neighboring tissues causing atrophy of ova and local circulatory dist urbances . Based on the tissue location of plasmodia and morphological character of the ma ture spores, the parasite in the present study might be a new species.