Approximately 900 km of 28-kHz echo sounding and 770 km of 1-kHz seismic-reflection profiles were obtained from Lake Turkana in northern Kenya between March and December 1984. The profiles penetrate as much as 60 m below the lake floor, representing perhaps the last few tens of thousands of years. They show that the predominant pattern of sedimentation is one of simple and rapid basin infilling. The profiles suggest an abundance of gas (probably methane) in the Holocene sediments, particularly near the major rivers and in the deepest basins where sedimentation rates are relatively high. High-angle growth faults, characteristic of extensional regimes, are common and indicate that faulting is active today, particularly in the central basin of the lake. A major erosional unconformity indicates that lake level was about 60 m lower than present lake level, probably just prior to 10,000 yr B.P., when unusually arid conditions prevailed throughout much of east Africa.