Marine geophysical investigations in the area of the Perth Basin lead to proposed changes in the structural control of the basin and of the structure within the basin. The main north-south graben structure appears to be crossed by a series of major faults which trend roughly north-northwest. A broadening of the basin to a width of just over 100 km (65 miles) to the west in the area between Perth and Harvey, which was earlier indicated by aeromagnetic results, appears to be produced by two such faults: The southern fault does not cross the graben but merges with it in the form of the Dunsborough Fault; the proposed northern fault crosses the graben to produce a break in continuity of the Darling Fault which marks the eastern margin of the basin. The northern fault appears to have caused a division of the main sedimentation axis of the Perth Basin into two near parallel axes within the widened section of the basin. The Dandaragan Trough which forms the eastern axis now appears to terminate at the Darling Fault between Pinjarra and Harvey. The western axis continues to the southeast into the Bunbury Trough. This suggests the presence of a basement and anticlinal ridge which may produce favourable traps for oil or gas within the basin. The western margin of the basin appears to be formed by an eastward-tilted basement with associated faulting which, in places, assumes major proportions. A tentative estimate of sediment thickness of about 5.7 km (18,600 ft) is obtained from a seismic profile near the axis of sedimentation on the extension of the Bunbury Trough. This figure is our preferred interpretation, but it would have to be reduced to about 3.5 km should a 5-km/sec layer turn out to be basement.The southward extension of the Darling Fault onto the narrow continental shelf appears to be observed with a throw of two km to the south of Pt. D'Entrecasteau.