The temporal variations in the taxonomic composition, biomass andrnphotosynthetic production of phytoplankton in Lake Bishoftu-Guda wererninvestigated for a period of one year, from November, 2004 to October,rn2005. Concurrently changes in some physico-chemical environmentalrnvariables were studied. Temporal variations in inorganic nutrients werernobserved with the maximum values cOinciding with a period of relativelyrnhigh rainfall at both stations. Lake's transparency was always high (ZSDrn> 1m). Since the lake was transparent due to low phytoplanktonrnbiomass and suspended inorganic particles throughout the studyrnperiod, the extinction of underwater light was low. The phytoplanktonrncommunity of this nutrient-poor and deep lake was dominated byrndinof/agllates during the dry and short rain periods before a shift inrndominance to diatoms occurred during the long rainy period.rnPhytoplankton biomass of Lake Bishoftu-Guda was found to vary from 4rnto 20 (mean=1O.8) mg Chi a m-3 at the central station and from 3.96 torn24.2 (mean=lO. 2) mg Chi a m-3 at the near-shore station. The depthprofilesrnof gross photosynthetic activity were of a typical pattern forrnphytoplankton with light-inhibition at the surface and with light-saturatedrnrates generally occurring at a depth of 0.9 m of the water column on allrnsampling dates. The light-saturated rates of photosynthesis (Amax)rnranged from 106 to 407 mg O2 (-33.1 to 127 mg C) m-3 h-I. A strongrnand positive correlation (r=0.76, p=O.OOO) was found between Amax andrnphytoplankton biomass. Biomass-specific rate of photosyntheticrnproduction at light-saturation (Pmax) ranged from 19 to 29 mg O2 (mg chirnarl h-I and was inversely related to phytoplankton biomass. Hourly integral photosynthesis (LA, g d m-2 h-Irn), which was obtained by thernGrid Enumeration Analysis ranged from 0.112 to 0.66 g O2 (-0.03 torn0.21 g C) m-2 h-I. There was a positive but weak correlation (0.36),rnP=O.OOO) between phytoplankton biomass and hourly integralrnphotosynthesis, while the correlation between LA and Amax was 0.83.rnDaily production rates per unit area (LlA g d m-2 efl), were estimatedrnfrom hourly integrated rates and the calculated daily integral productionrnvalues ranged from 1.01 to 5.98 g O2 (-0.32 to 1.87 g C) m-2 efl. Therntemporal variations in species composition, biomass and photosyntheticrnproduction of phytoplankton are discussed in relation to physicochemicalrnfactors.