The effect of a magnetic field upon the properties of a plasma created by CO2laser‐induced gas breakdown in helium has been investigated. Spatially and temporally resolved temperatures in the 7–20‐eV range were observed using the spectral‐line ratio technique, in particular the He II 468.6‐nm line‐to‐continuum ratio. The continuum measurements were also employed to obtain relative values of electron density. The temporal dependences of electron density and temperature indicate that energy is lost from the plasma by an adiabaticlike expansion. The presence of the magnetic field significantly decreases both the energy and density loss rates.