In conical flows the velocity, pressure, and density do not vary on lines through the vertex of the cone. In this paper it is shown that for interferograms of general conical flows &dgr;(y, z)/z=f(y/z), whereyandzare any set of Cartesian coordinates with origin at the image of the vertex of the cone, and &dgr; is the fringe shift aty, z. Thus, for strictly conical flow, a graph of &dgr;(y, z)/z versus y/zshould be a single curve. This suggests a test for approximate conicity that requires very little computation. This test is applied to interferograms obtained from a number of approximately axisymmetric flows at various Mach numbers about cone‐cylinders in free flight. Plotted fringe shift data from the region near the nose fall into a narrow band, an indication of approximate conicity. They also closely check the corresponding theoretical fringe shift curve calculated for Taylor‐Maccoll flow.