The collisionless flow of a laser‐produced barium plasma through low‐pressure photoionized xenon in the presence of a uniform, transverse magnetic field is studied. Deceleration of the barium is observed, and attributed to magnetically induced instabilities. Interaction close (4 cm) to the target is observed. The front thickness is observed to vary inversely with field strength. Spontaneous emission of 4554.03 A˚ radiation increases by more than an order of magnitude when theBfield and background gas are both present, indicating significant electron heating. Theory shows that these observations can be explained by the modified two‐stream instability and the magnetized ion–ion instability.