Time‐delayed correlation measurements with a CO2laser, multichord plasma interferometer show density perturbations propagating along the local magnetic field in the edge region of the ZT‐40M reversed field pinch. The observed speed is consistent with ion‐acoustic propagation. Such measurements of field‐parallel waves with long wavelength (&lgr;∥≳several centimeters) may provide a quasilocal, quasi‐cw diagnostic of the sound speed in many low &bgr; magnetically confined plasmas, useful in estimating plasma temperatures.