The results are given of an experimental investigation of the flow in the initial section of a turbulent underexpanded jet exhausting from a profiled nozzle with Mach number M a = 2.56 at the exit into a parallel stream with Mach number M∞ = 3.1. Analysis of the results of measurement of the fields of the total head p0 and the stagnation temperature T0 in conjunction with results of calculation of a jet of an ideal gas make it possible to construct the velocity profile in the mixing layer of the underexpanded jet in the parallel supersonic flow.