SYSTEM design requirements frequently necessitate a prek^diction of the structure of the plume formed by the exhaust of an underexpanded nozzle flow into the surrounding environment. The difficulty of this prediction is a function of the degree of underexpansion involved, the downstream distance over which the plume is to be defined, arid the motion of the environment relative to the plume. A condition of particular interest in flight situations where exhaust plume signal interference and/or infrared signature must be assessed, involves the exhaust of a moderately under-expanded flow into a co-flowing airstream. In such cases the structure of a plume must be defined in regions dominated by gas dynamic effects in the vicinity of the nozzle exit plane, and by the effects of turbulent mixing, and frequently chemistry, further downstream. Although no practical means are presently available for the general computation of such complex flowfields, efforts continue in the development of flow models which attempt to account for the influence of those effects considered to have a major influence on the plume structure." This Note concerns one particular aspect of the prediction problem, namely, the axial location of the Mach disk in an underexpanded jet in a co-flowing airstream.