Hitherto it has appeared that faulting is rather rare in Malaya, despite other indications of strong earth movements. Now, due largely to a wide diversity of lithology within a small compass, considerable faulting has been demonstrated in the Baling area in the northwest of the country. This comprises two complementary fault sets disposed in a typical Anderson-type wrench-fault pattern. The displacements, however, although essentially strike slip in nature, are in the opposite sense to that implied by the disposition of the faults. It seems that two phases of compression have occurred, forces being aligned north and south in the earlier phase, in which the faults were initiated, and east and west in the later one when the present-day displacements were effected. The principal fault of the Baling area, the ok Bak fault, has been mapped for some 51 miles and postulated for a further 52 miles in NW. Malaya. It exhibits sinistral (left-lateral) displacement of 32-36 miles. Continuing the line of the fault through the Thai-Malay peninsula, it is seen to coincide with a number of geological and morphological discontinuities which can be largely resolved by restoring the indicated offset along the Bok Bak fault. It therefore seems that the fault is at least 650 miles long. In common with many other large wrench faults, the date of the inception of the fault is obscure. It appears, however, that the main (sinistral) movement took place in the Mesozoic orogeny (Jurassic to Cretaceous). There is evidence of wrench faulting in other parts of Malaya, and it is likely that this fault style will prove to be more widespread here than previously suspected.