The incidence of Cryptosporidium nasorum was investigated among 120 Tilapia zelii obtained from a fish farm, King Mariote, Egypt. Intestinal smears were prepared, fixed and stained with Modified Zeihl-Nelson (M.Z.N).Twenty percent of fish were infected with C. nasorum. Fifteen fingerlings of Tilapia zelii fish were each infected experimentally with 50,000 C. nasorum oocysts and ten fingerlings were each given 100,000 oocysts. Oocysts were detected by mucosal smears at 4 to 5 days postinfection (dpi). Schizonts were identified at 2 to 3 dpi and zygotes were found 3 to 4 dpi. No oocysts were detected by examination of smears and no endogenous stages were found in non-infected Tilapia zelii. Histopathological examination of fifteen mice (Two days old) experimentally infected with 100,000 C. nasorum oocysts isolated from infected T. zelii fish failed to induce infection. The same result was obtained in five (ten days old) mice inoculated with 100,000 oocysts. This study confirmed the tropism of Cryptosporidium species to the jejenum of the T. zelii fish. It also showed that infection of mammals with the fish Cryptosporidiosis did not ocuur.