The average of 8 heat-flow measurements in the western half of the Cayman Trough is 2.07 ± 0.25 HFU, markedly higher than the averages of 1.46 ± 0.19 HFU (n = 9) and 1.38 ± 0.19 HFU (n = 3) obtained on the Yucatan Basin and Cayman Ridge, respectively. There is a tendency for the highest heat-flow values to be situated in the deepest areas of the trough. No systematic variation of heat flow with distance along the trough was observed. The existence of a long, narrow zone of uniformly high heat flow along the floor of the Cayman Trough, along with other geophysical data, suggest a tectonic origin for the trough by extension normal to the axis of the trough and/or by strike-slip faulting related to the eastward movement of the Caribbean lithospheric plate relative to the Atlantic plate. The mean heat flows through the Yucatan Basin and Cayman Ridge are nearly equal to the average world heat flow. Seismic profiler data and piston cores from the Cayman Trough and Ridge and in the Yucatan Basin show that the Cayman Trough is a geologically young feature, probably having originated since the early Tertiary, when the trough and adjacent ridge developed simultaneously. Subsequently, the western end of the trough has received terrigenous sediment from a source located near the Gulf of Honduras, rather than from the Yucatan Basin or Honduras. The central and eastern parts of the trough have been and remain isolated from any major sediment sources. Tectonic activity in the trough has been largely restricted to the margins, as evidenced by the location of the deepest basins, fault structures, and seismicity along the southern margin of the trough west of 83° W. and along the base of the Cayman Ridge east of 81° W.