The stability of pipe Poiseuille flow to azimuthally periodic disturbances is examined experimentally. The disturbance is generated in the fully developed flow region to avoid extraneous effects of possible entrance flow instability. A Reynolds number spectrum to above 3200 was covered with a frequency range from13to1kc/sec. The peak disturbance velocity amplitude was maintained at a value less than 2% of the maximum steady‐state velocity in the pipe. Some measurements of spacewise changes in velocity and disturbance wavelength were made as well as radial surveys of disturbance amplitude and steady‐state profiles. The results indicate that the stability is both frequency and Reynolds number dependent to the first mode of azimuthal periodicity, with a minimum critical Reynolds number occurring at about 2150.