Seasonal and spatial variations of zooplanktoR Â·in relation to physico-chemical,rnvariables and phytoplankton production in Lake Beseka were studied fromrnOctober 2004 to September 2005. The measurements were made on monthlyrnbasis. The lake shows temporal variations in temperature (24 -31Â°G),rntransparency (0.42-0.63 m) and turbidity (27 -88 NTU). Conductivity (6332 -rn7438jJScm,1), total alkalinity (31- 41 meq/l) and pH (9.1-9.5) are higher thanrnthose of most other Ethiopian rift valley lakes. The present measurements ofrnphytoplankton production (166 -273 mg O2 m'3h(1) and biomass (7-17 mg chi arnm'3) suggest that the lake is not productive as other saline lakes. One factorrnanalysis of variance (A NO VA) and Pearson correlation were used to estimate thernlevel of interaction of physico - chemical and biological factors with zooplanktonrndynamics. All major zooplankton groups showed temporal variations inrnabundance, with highest densities during wet period (July to September) andrnminimum densities during dry period (November to April). The cyclopid copepodsrnwere the dominant group and made up almost 64% of the total zooplanktonrnabundance. Rotifers and cladocerans also contributed 19% and 13%,rnrespectively, of the major total zooplankton abundance of the lake. The results ofrnthe Pearson correlation demonstrated that population dynamics, of the majorrnzooplankton groups in Lake Beseka appear to be mainly governed by biologicalrnand environmental factors, their densities fluctuating along with primaryrnproduction, phytoplankton biomass and nutrients. Among environmental factors,rnhigh temperature and rainfall appear to have promoted a higher density of eggrnproduction and total zooplankton number, respectively. Turbidity affected therncladoceran population negatively.rnKey words: Lake Beseka, zooplankton, temp?ral dynamics,