The main aim of this study was to investigate the clinical and postmortem findings, seasonal prevalence, and histopathological alterations 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 recorded in summer. Six (40%) out of the infested fish had Myxobolus cysts in only one ovary, meanwhile, the reminder (60%) of infested fish had both ovaries infested. Also, intensity of infestation gradually increased over winter and was maximal in early spring, but abruptly declined in summer. Microscopic examination of plasmodia showed numerous typical Myxobolus spores at various 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 revealed that Myxobolus plasmodia were encapsulated within a thin connective tissue layer of host reaction.  Myxobolus cysts compress neighboring tissues causing atrophy of ova and local circulatory disturbances. Based on the tissue location of plasmodia and morphological character of the mature spores, the parasite in the present study might be a new species.