A comparison was made of plasma density measurements obtained using cylindrical Langmuir probes and a high‐sensitivity microwave interferometer in magnetic fields between 1 and 7 kG. The experiment was carried out in a current‐free stream of ionized hydrogen with electron densities ranging from3×109to4×1011 cm−3. A set of probes of different sizes was used so that the ratio of probe diameter to ion gyroradius2rp/&rgr;icovered the critical range from 0.2 to 5 or more. It was found that (1) the shape of the probe characteristics was not affected very much by the presence of the strong magnetic field; (2) the electron temperature derived from the slope seemed to be the true temperature; but (3) the apparent ion density inferred from the characteristic using Laframboise's calculation, which applies in the absence of magnetic fields, was consistently low by a factor which depended primarily on the ratiorp/&rgr;iand seemed nearly independent of the Debye length. This reduction factor was considerably larger in the streaming plasma than in a stationary one, presumably because of the wake or “shadow” cast by the probe whenrp/&rgr;iis not negligible.