Qualitative sampling and relative abundance of phytoplankton of Apodu reservoir, an important tropical African drinking water reservoir in Malete, Nigeria was analysed monthly from the three stations for one year between January 2018 and December 2018 spanning both rainy and dry seasons, using a plankton net. Identification of the phytoplankton was done under a digital binocular compound microscope equipped with a camera. The phytoplankton were identified to generic level using standard taxonomic identification keys. Six classes of phytoplankton namely, Bacillariophyceae, Chlorophyceae, Cyanophyceae, Zygnematophyceae, Coscinodiscophyceae, Xanthophyceae were recorded in the reservoir. Thirty-five (35) genera were found in the phytoplankton classes with Chlorophyceae having the highest of thirteen genera, while Coscinodiscophyceae was the least with one genus. Microspora had the highest relative abundance of the total phytoplankton, while Stephanodiscus had the least relative abundance of the total phytoplankton population. The phytoplankton classes were present in all the three stations and highly abundant in the rainy season. The assemblages of the various classes of phytoplankton in the reservoir was attributed to the optimal ranges of the physicochemical factors of the water. The heterogeneity of the phytoplankton assemblage enabled high primary production in the reservoir and this will support a high fish production in the reservoir.