A multiple‐pass Mach‐Zehnder laser‐illuminated interferometer has been constructed and used to measure the electron density profile of plasmas produced by a self‐confined propagating electron beam. Spatial and time resolved data were taken perpendicular to the electron beam axis at two longitudinal positions. Densities of 1016cm−3have been seen in air between 0.15 and 0.3 Torr as early as 150 nsec after the electron beam entrance. Peak densities of 1017cm−3were typically reached at around 2 &mgr;sec. Plasma compression was observed under some conditions.