The studied area is located 335 Km. southwest of Addis Ababa in the southwestern Ethiopian plateau. Over half of the map-area is underlain by thick basalt flows inter layered with tuffs and volcanoclastic sediments. The Jimma Volcanics, which is mainly composed of massive rhyolite in thick flows alternating with trachytes, tuffs, ignimbrites and subordinate • • basalts, underlain small area in the eastern and northeastern part of the basin. Based on the hydrometeorologic data collected for the last 38 years an attempt ha~ been made to calculate the hydrological parameters like precipitatior, evapotranspiration, and runoff. The annual mean rain~All in the basin is 1476.89 mm. The values of the annual ,lotential e,vaportanspirtion and actual e,v apotranspiration are 837.51 mm and 822.29 mm respectively. The only runoff that leaves the studied area comes through Boye river and is 90.77 million cubic meter of water. The amount of water which is actually available to recharge the groundwater circulation within the hydrogeological basin is 51.14 million cubic meter. The hydrogeological character of the outcropping lithotypes was discussed with particular reference to infiltration and their water bearing capacity. The hydrogeology of the area varies in different parts of the basin. There is a great potential of groundwater in the valleys which is partly filled with alluvial sediments while a less significant amount exist within the volcanic rocks forming the uplifted borders of the valleys. Determination of the hydraulic parameters like transmissivity and permeability have been carried out. Due to the absence of observation well nearby a discharging wells it was impossible to calculate the coeff icient of storage, a significant ~arameter which measures the volume of water the aquifer releases~~ take into storage. For basaltic aquifers a transmissivity value of 18.39 m2/day and a permeability value of 3.31 x 10-4 cm/sec. have been calculated. A transmissivity value of 1.89 m2 /day. and a permeability value of 1.62 x 10-5 cm/sec. have been co.mputed for both alluvial and rhyolitic aquifers. Because of the lack of bore w~lls in the southwest, south, and southeast lower most part of the basin a thorough study of the actual variation of groundwater level was not possible. For the other part of the basin, appropriate groundwater level contour lines was constructed on the basis of the available static level data. Proper chemical analysis of both the groundwaters and surfaces water carried out in the basin show low total dissolved solids. All of the samples have less than 1000 mg/l dissolved solids. The analysed samples also compared with the acceptable standard for human, agricultural, and industrial use