Goldfish, common carp and Nile tilapia were sampled between September 2002 and May 2003 to investigate lesions induced by viral diseases and ectoparasites. Goldfish exhibited neoplasms, Dermocystidium sp., Ichthyophthirius multifiliis (Ich), Trichodina reticulata, Lernaea cyprinacea and systemic infections. Neoplastic and systemic infections in goldfish were associated with viral infection. Oreochromis niloticus and Cyprinus carpio were mainly infested with several species of Trichodina and monogeneans. The presence of Dermocystidium sp. in carp, and viral particles in the liver and spleen of O. niloticus and C. carpio, was uncommon. Microscopically, intracytoplasmic inclusion bodies in the hepatic cells and lymphocytic infiltration in other internal organs of the three fish species were seen. Ultrastructural study showed enveloped viral particles in some cells in all fish species. The recorded parasitic infections were associated with degenerative, necrotic, inflammatory and proliferative changes in the skin and underlying muscles. Imported ornamental goldfish may be a source of infection to carp and native tilapia. Histopathology and transmission electron microscopy demonstrated the etiology and pathogenesis of infection and the preliminary diagnosis of viral infection, which is not common in Egypt. It is recommended that goldfish be excluded from edible-fish aquaculture ponds. Strict regulations regarding the transfer and quarantine of imported fish should be implemented.