A stabilized laser interferometer is proposed with two beams as the light source. The fringe shift for a 0.63 &mgr;m beam of a He–Ne laser is used to stabilize the interferometer against the effect of mechanical vibrations via a feedback controlled speaker coil, while another beam of 3.39 &mgr;m, for which consequently the effect of the mechanical vibrations is excluded, is used to measure the plasma density. A stability of ∼1/500 of one fringe for 0.63 &mgr;m is obtained during a long period for frequencies lower than a few Hz. The stability for higher frequencies is limited to ∼1/30 of one fringe for 0.63 &mgr;m, which correspondes to ∼1/200 of one fringe for 3.39 &mgr;m, by the acoustic noise picked up by the speaker coil. Furthermore, the total accuracy is limited by the detector noise to ∼1/60 of one fringe for 3.39 &mgr;m, which corresponds to a line electron density of ∼5×1014cm−2. The detector noise may be reduced by cooling the detector. The advantage of this technique over the single‐laser technique is that the frequency response of the interferometer extends down to zero frequency. The interferometer is tested with the measurement of a plasma in a dynamic magnetic arcjet. Since the effect of the neutral gas background is reduced in the present interferometer, the application has an advantage for the diagnostics of plasmas produced in high pressure gases.