In 1977 a wide-aperture seismic network of land and ocean bottom stations (OBS) was operated for 6 weeks in the southem New Hebrides island arc. Data on the spatial distribution and mechanisms of small events recorded by this local network were integrated with worldwide observations of moderate to large size New Hebrides earthquakes of the past 17 years to study the structure and deformational processes in the New Hebrides subduction zone. Surprisingly, small differences (less than about 10 km) were found between the locations of shallow- and intermediate-depth earthquakes as located by the temporary local network and by the Intemational Seismological Centre using worldwide stations. Analysis of the P wave travel time data from the OBS station closest to the trench shows limited evidence for a high-velocity zone (about 5% higher than the surrounding mantle) associated with the descending plate. The thrust zone between the descending and upper plates is defined mainly by the spatial distribution and focal mechanisms of moderate to large size events and their aftershocks, but it is not very well defined by he spatial distribution of small size events located in this study. Shallow seismic activity located beneath trench and beneath the interplate thrust zone indicates a lower limit of about 40 km for the thickness of the seismically active part of the descending plate beneath the thrust zone. Focal mechanisms of moherate to large size earthquakes located within the upper plate and geological observations on the islands suggest that the upper plate in the region of Erromango and Tanna is divided into a series of blockskhat are differentially uplifted along mainly NW-SE striking faults. During the OBS experiment several shallow events were well located beneath the Coriolis trough, a riftlike feature located to the east of the vol canic arc. Well-determined depths of these events are between 11 and 22 km. The well-located intermediate-depth events define a 20-km-thick Benioff zone that has a dip of about 70°. The spatial distribution of events within the descending slab is very strongly clustered in a persistent pattern that is seen in both the short-term sample of small events determined by the local network and the long-term sample of locations based on worldwide data. Several focal mechanisms of moderate to large size intermediate-depth events show components of lateral extension along the strike of the arc and some show components of lateral compression, both of which could be interpreted as the result of a lateral bending of the descending plate in the part of the southem New Hebrides arc where the trench begins to curve around to the east.