Legedadi Reservoir, a major source of drinking water for Addis Ababa, is experiencing algal bloom frequently. A study on Physico-chemical and biological (phytoplankton and zooplankton) parameters could give some clue on alternative ways of controlling the algal blooms. Some limnological features of the reservoir were studied to this end. Samples were taken monthly at three stations from May to October 2017 on both parameters. All the data were collected and analyzed using standardized sampling techniques and methods. A 15Î¼m and 30Î¼m mesh size plankton nets were used for phytoplankton and zooplankton sampling, respectively. rnThe results indicated that the reservoir is remarkably fresh, (0.08 â€°), turbid, moderately warm (19 oC), and less oxygenated (5.93 mg L-1). It is one of the most turbid waters (239 - 936 NTU) in Ethiopia with shallow Secchi depth (0.098 m) and with high values of algal nutrients and alkalinity. All abiotic parameters considered in the study did not show spatial variation except pH and sulfate (SO4), which also exhibited significant temporal variation (ANOVA, Î± = 0.05). rnA total of 14 algal species belonging to four taxonomic classes were identified. Microcystis species and filamentous, Anabaena and Planktolyngbya species were the most dominant ones in the reservoir. Phytoplankton biomass as Chl-a ranged from 1.1 to 14.3 Î¼g L-1with a mean value of 5 Î¼g L-1. The minimum value was recorded in the rainy season (June â€“ September) while the maximum was in late October. The photosynthetic productivity of phytoplankton of the reservoir as the rate of photosynthesis (Amax) ranged from 58 mg C m-3 h-1 to 128.5 mg C m-3 h-1. rn26 typical tropical species of zooplankton were identified from the reservoir, of which 21 species belonged to Rotifera while 4 species belonged to Copepoda and one species (Daphnia barbata) to Cladocera. Total zooplankton abundance (27, 470 ind m-3) was dominated by copepods (53 %) followed by rotifers (47 %). Among the rotifer abundance(12,844ind m-3) Brachionus sp., Filinia opoliensis, Keratella tropica, and Polyarthra remata were the most abundant and persistent while Metadiaptomus colonialis and Paradiaptomus (Lovenula) falcifera were the most abundant and persistent copepods. rnThe size of zooplankton was of typical tropical small nature. Zooplankton biomass ranged from 0.43 to 94.2 mg DW m-3 with a mean value of 21.5 mg DW m-3. Of the total standing biomass of zooplankton, 97 % (21 mg DW m-3) was contributed by copepods, 2 % (0.4 mg DW m-3) by D. rnrnbarbata, and 1 % (0.1 mg DW m-3) by rotifers. 40.8 % of the mean zooplankton biomass was contributed by nauplii of copepods (both calanoid and cyclopoid) while Filinia opoliensis accounted for 66 % of the total rotifer biomass. rnZB to Chl-a ratio for Legedadi Reservoir is of high magnitude (4.3) indicating that zooplankton biomass is capable of controlling phytoplankton biomass in the reservoir. However, the dominant zooplankton in the lake are Rotifera with small body sizes and incapable of grazing on the large inedible algae. Hence, although zooplankton is maybe capable of controlling the frequently observed algal bloom in the reservoir, most of the bloom is contributed by inedible larger algae such as Microcystis, Anabaena, and Planktolyngbya species. Hence, the biomanipulation option of algal control with zooplankton grazing does not appear to be promising. A remote option would be to investigate herbivore fish that can selectively graze on these algal species in the reservoir.