Parasitological survey on myxosporidia protozoal infestation in Nile Catfish, Clarias lazera was conducted during the period of 1995-1997. The study showed that 171 fish out of 206 (83.1%) were infested with more than one species of myxosporidia. The highest prevalence of infestation was observed in Autumn (93.6%) and decreased to 69.2% during summer. The secondary Respiratory Organ (SRO), posterior kidneys, livers spleens and ovaries of infested and non-infested fish were fixed in Bouin's fixative, processed and examined by light microscopy. Histopathological alterations induced by Henneguya branchilis in the SRO, Myxobolus lazeri in the posterior kidneys, Myxosoma clarii in the livers, and Myxobolus clarii in the spleens and ovaries were described. Infested SRO, posterior kidneys and ovaries were fixed in 5% cold glutraldehyde, dehydrated, embedded in Epon 812 and examined by Joel CXII 100 transmission electron microscopy. The significanse of these results regarding the host-parasite relationship was discussed.