Earthquake data mainly between May and July 2001 are used in this study of the seismicityrnof the Main Ethiopian Rift system. The major source of data is the IRIS/PASSCALrn(Incorporated Research Institutes of Seismology/Program for the Array Seismic Studies ofrnthe Continental Lithosphere) Broadband Seismic Experiment recently conducted in Ethiopiarn(Nyblade and Langston, 2002). This experiment was operational between 2000 and 2002.rnHypocentral parameters for 144 earthquakes are determined using the location programrnHypoinverse-2000. The locations stability is tested by relocating the earthquakes using therndouble-difference algorithm. The distribution of epicenters in this study shows high seismicrnactivity around 90N and 400E, and 9.50N and 39.50E during the study period. Thesernepicenters are close to the N-S trending Ankober border fault and Dofen volcano. Localrnmagnitudes are estimated for the reported events. Fault plane solutions are determined forrnfive selected earthquakes using the FOCMEC program and show pure normal faultingrnmechanisms. Four of these mechanisms, which were located near the Ankober Border Fault,rnshow N-S trending fault orientation while the fifth solution for an earthquake from thernwestern plateau has nodal planes trending NW-SE. On the other hand, three separate bvaluesrnare estimated using both the least-squares method and the maximum-likelihoodrnmethod for the whole rift system and the high seismicity (Ankober-Dofen) area. For thernwhole rift under study, b-values of 1.059Â±0.13 were obtained using the maximum-likelihoodrnmethod and 1.22 using the least-squares method. On the other hand, b-values of 1.17Â±0.16rnand 1.37 using the maximum-likelihood and least-squares methods, respectively, are alsorndetermined for the highly seismic Ankober-Dofen region during the study period. Byrnincluding magnitude data from another source for the year 2001 a separate b-value is alsornestimated for the region which experienced higher seismic activity - a maximum-likelihood estimate of 0.81Â±0.05 and a least-squares estimate of 0.91 is obtained. The b-valuesrndetermined based on the data only from this study are indicative of a low stress region. Thernresults could be taken as an indication that seismic energy was released in the region duringrnthe study period mainly in the form of small magnitude earthquakes. The relatively low bvaluernestimated by including data from another source shows a relatively high stress for thernregion. The low b-value estimated by including data from another source may be due to thernwide time period considered indicative of the temporal variation of seismicity rate in a givenrnregion.