The linearized spatial stability of adiabatic flat plate flow to the first mode of oblique disturbances is computed numerically, using finite difference techniques, in the Mach number range M=1.6 to 6.0. The most unstable wave angle &psgr; is found in the range &psgr;=46° to 60°. Stability maps, in the form of curves of constant spatial amplification rate, are presented on the frequency‐Reynolds number diagram. The criticalx‐Reynolds number is found to decrease monotonically with M, and is best fit with the expression R1/2x=579.34M−1.18. This decrease is found to correspond with the outward displacement of the minimum critical layer and the ‘‘inflection’’ point,yat (U′/T)′=0, fromy=0.248 and 0.231 at M=1.6 to 0.754 and 0.842 at M=6.0. No transition from viscous to inviscid instability is found with increasing Mach number, rather viscous instability persists to M=6.0. Some of the results agree with those obtained earlier by Mack, but others differ, particularly computations for M>3.0.