A microearthquake network with six stations and a diameter of about 50 km has been in operation in the Stiegler's Gorge area (Rufiji Basin), Tanzania, between 1978 and 1981. The network, which used radiotelemetry and digital recording at 30 Hz based on a triggering system, has recorded a large number of teleseismic and regional earthquakes in addition to several thousand local ones within the network. Most of the locally recorded earthquakes are in magnitude range 0--4 measured on a duration magnitude scale which has been developed for the area, a scale which also satisfactorily covers regional earthquakes with distance up to 25¿. An active and well-defined seismicity zone has been delineated in the Stiegler's Gorge area, striking N140¿E and dipping 45¿NE, with events distributed between 0 and 40 km in depth. The time and space development of the local seismicity shows a fairly stable pattern, and a detailed focal mechanism study within the most active zone shows a predominance for normal faulting, with eastwest extensional movements. This is consistent with results from local geological studies, even though there is no clear correlation between the seismicity and locally mapped faults, and it also consistent with our knowledge of the regional tectonics of the East African Rift System.