A one month microearthquake survey was conducted at Iheya Knolls in the middle Okinawa Trough in 1992 to reveal microearthquake characteristics of back-arc rifting. We deployed five ocean bottom seismometers and observed over 1700 events during 30 days. Hypocenters were determined for 48 earthquakes with good resolution. From this survey, we conclude that the survey area with 100 km¿100 km has high seismicity, many earthquakes occur in the western part of Iheya Knolls, and few occur in the central part. The earthquakes are distributed in an en echelon shape and throughout the entire depth of the crust. There is no evidence for the existence of transform faults, suggesting that the middle Okinawa Trough is in a stage of back-arc rifting. ¿ American Geophysical Union 1994