A temporary network of up to 23 short-period seismic stations operated on the Reykjanes Peninsula during the summer of 1972 and recorded an earthquake swarm consisting of more than 17,000 events. The area studied is the immediate landward extension of the Mid-Atlantic Ridge, and seismic processes are probably similar to those on submarine parts of the ridge crest. The Reykjanes Peninsula forms the transition between the Reykjanes Ridge and the south Iceland transform fault. The peninsula is essentially a leaky transform fault, and focal mechanism solutions are of both normal and strike-slip types. The 12-km-long seismic zone active during the 3-day swarm is defined by 2514 hypocenters, which are mostly between 2 and 5 km deep. Individual structures within the seismic zone include several seismic lineations or faults trending obliquely to the zone. Also visible are two aseismic zones, which may be eithr rigid blocks or low-rigidity regions, such as magna chambers. Thus the observed seismic zone is not a simgloe fault; instead it appears to be the shallow expression of a deeper-seated and aseismic deformation zone. In time, the seismic events cluster together into subswarms lasting a few hours and migrate laterally at sppeds of 1--2 km/d. The energy released is equivalent to that of a single event of Icelandic local magnitude 4.9, and the total fault area is close to that ejpected for a single earthquake of this size. Although the high geothermal area, the observed earthquakes do not locate in or outline the hydrothermal upwelling zone visible at the surface. This contribution is the first study to reveal the details of faulting during an ocean ridge earthquake swarm, both in space and in time.