About 2400 line km of magnetic and bathymetric data were collected between the western margin of Jamaica and the eastern margin of Belize covering the Cayman Trough. The echograms show rugged topography between 80 degrees W and 85 degrees W longitude. The mid-Cayman spreading centre is characterized by a rift valley with 4000 m depth. Sea-floor spreading-type linear magnetic anomalies have been observed in the Cayman Trough. A synthetic sea-floor-spreading model is generated across the spreading axis with crustal block thickness of 1 km and a susceptibility of 0.002 cgs units. The anomaly numbers 1 to 8 have been identified on the west and anomalies 1 to 5A on the east of the spreading axis which is located around 81 degrees 39'W. The half-spreading rates of 10 mm yr sup(-1) (0-2.47 Ma) and 20 mm yr sup(-1) (2.47-27.74 Ma) are estimated in the Cayman Trough. The crust west of anomaly 8 appears to form the Continent Ocean Boundary off Belize. It is suggested that the opening of the Cayman Trough was initiated prior to late Oligocene