Dynamics of phytoplankton and physico-chemical parameters of water quality werernstudied in Geffersa Reservoir from October, 2009 to June, 2010 using samplesrncollected at bi-weekly intervals. Secchi depth (vertical visibility) varied from arnminimum of 0.20 of the minor rainy season to a maximum of 0.66m during the dryrnperiod. Turbidity, which fluctuated between 7.2 and 189 NTU, exhibited a seasonalrnpattern, which was more or less similar to that of phytoplankton biomass measuredrnas Chl a (r= 0.649, r2= 0.415 at p= 0.0070). The concentrations of NO3-N, NH3 + NH4+-rnN and Soluble Reactive Phosphate (SRP) varied from 10.0 to 300, 146 to 866 and 29rnto 201 Î¼g L-1 respectively while Molybdate-reactive silica ranged from 2.5 to 13.4 m L-rn1. Turbidity, TDS, and other chemical parameters were found to be far below thernupper permissible limits for drinking water supply sources. The phytoplanktonrncommunity whose species diversity was found to be low was primarily constituted byrngreen algae, blue-green algae and diatoms, the contributions of the species-rich greenrnalgae ranging from 1 to 95 % and with major species belonging to the generarnCrucigenia, Kirchneriella and Ankistrodesmus. The second and third most abundantrnalgal groups were the blue-green algae and diatoms, which were dominated by twornspecies of Microcystis, M. aeruginosa and M. panniformis. and by a species of thernfilamentous genus Melosira, M. agassii, followed by Fragilaria capucina andrnRhopalodia gibberulla. Among the flagellate groups, dinoflagellates were the mostrnimportant, with Peridinium cinctum and P. bipes as the quantitatively most importantrnspecies. Among the cryptomonads, Cryptomonas marsonii and C. ovata were found inrnlarge numbers, Chl a biomass of phytoplankton averaged 12.41 Î¼g L-1 and variedrnfrom a minimum of 2.29 Î¼g L-1 to a maximum of 40.67 Î¼g L-1 with its fluctuationsrnbeing correlated more with ammonia +ammonim (r =0.662, r2 = 0.44 at p=0.0045)rnthan with nitrate (r=-0.4093, r2= 0.1675 at p= 0.1028) or phosphate (r= 0.435,rnr2=0.289 at p= 0.0800). Rotifers were the most abundant and species-richrnzooplankton group with mean percentage contributions of 66.82 while therncontributions from copepods and cladocerans were 24.51% and 8.67% of the totalrnabundance of the zooplankton respectively. The causal relationships amongrnphysicochemical and biological variables are discussed and recommendations made.