Aim of the work: the present work aimed to study the comparative histology of some fresh water fishes inhabiting the Nile River in Egypt. These fishes have different dial activities; Oreochromis niloticus is diurnal,while Clarias gariepinus, Bagrus bajad and Chrysichthys auratus are nocturnal. Results: the retinae of all studied fishes (O. niloticus,C. gariepinus, B. bajad and Ch. auratus) are composed of ten layers. These layers are the inner limiting membrane (ILM), nerve fiber layer (NFL),

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... nglion cell layer (GCL), inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), outer nuclear layer (ONL), outer limiting membrane (OLM), photoreceptor layer (PL) and pigmented epithelial layer (PE). The histological structure showed three types of the photoreceptors (single and double cones as well as single rods) in Oreochromis niloticus. The pigment epithelium layer in C. gariepinus, B.bajad and Ch. auratus completely hides the photoreceptors. A considerable variation was recorded between the different studied fishes in the whole retinal thickness as well as their retinal layers. Conclusion: Whole retinal thickness in O. niloticus (diurnal activity) is large in comparison with that of the other three siluriform fishes (nocturnal activity).