A detailed gravity survey of the Main Ethiopian Rift Valleyrnbetween latitudes 70 N and SON, has shown that there is a broadrnrelative positive Bouguer anomaly over the whole of the Rift floor,rnand this anomaly is superimposed on the regional, broad negativernanomaly of the Ethiopian and Somali Plateau. The broad relativernpositive anomaly over the rift floor is between 60 and 100 km widernand has an amplitude of 30 - 60 mga!. On the broad relative positivernanomaly over the rift floor are superimposed other short-wave lengthrnrelative positive anomalies which seem to be situated along therndisplacement lines of the Wonji Fault Belt. There are also muchrnnarrower relative positive anomalies along the margins of the riftrnother than those associated with the 'Ionji Fault Belt.rnIn their interpretation of the gravity minimum near the centerrnof profile C (page 47), Searle and Gouin (1972) suggested that thisrngravity minimum is due to low density lavas of the Aluto volcano andrnthey further a.8sociated the volcano with a small negative anomaly.rnOn the contrary the present survey shows that the volcano isrnassociated with a relative positive anomaly of magnituderncomparable to the short-wave length rela.tive positive anomaliesrnalong the displacement lines of the Vionji Fault Belt.rnDue to both, denser spacings of gravity stations and qualityrnof data, it is believed that the present gravity survey of the MainrnEthiopian Rift between latitudes 70N and SON defines morernaccurately the location of the gravity anomalies (both the negativernand the positive anomalies). The priliminary interpretations ofrnthe gravity anomalies made in this work are corroborated by borernhole data Wigs. 10, 11 ) and surface geology (F.ig. 2) of the studyrnarea. Furthermore, inferred density measurements on surface rocksrnand Cores from production bores in the Olkaria geothermal field ofrnKenya (Table 2) have been utilized