This study presents the phytoplankton and periphyton diversity of River Okhuo, Edo State, Nigeria from July-December, 2002. Phytoplankton samples were collected using a 55 μm mesh net while periphyton were collected by scrapping wood, stones, leaves and branches and both were preserved separately in 4% formalin. Camera Lucida drawings were taken and a total of 106 species, with more periphyton than phytoplankton were identified using monographs and research publications. The phytoplankton assemblages comprised of Bacillariophyta (43.94%) > Chlorophyta (42.42%) > Cyanophyta (6.06%) ≥ Euglenophyta (6.06%) > Rhodophyta (1.49%) while for the periphyton, distribution of species were: Chlorophyta (57.83 %) > Bacillariophyta (25.30%) > Cyanobacteria (10.84%) > Euglenophyta (4.82%) > Rhodophyta (1.21%). Chlorophyta was dominated by desmids, which were largely represented by species of Cosmarium and Closterium. The Bacillariophyta was represented by two (2) orders: Centrales and Pennales. Cyanobacteria was grouped into five families: Chroococcaceae, Nostocaceae, Rivulariaceae, Scytonemaceae and Oscillatoriaceae. Euglena (5) and Phacus (2) of Euglenophyta were recorded while Batrachospermum vagum was the only representative of the Rhodophyta.