In papers published previously, we discussed the oscillation wavelength shift of a semiconductor laser in a magnetic field at room temperature. Observations were carried out by means of a monochromator that was able to measure the oscillation wavelength shift in a steady state only and that was subject to a certain degree of measurement error. In this work, we used the beat note between two semiconductor lasers as a means of observing the wavelength shift. This method reduces the overall measurement error to about one tenth that obtained when using the monochromator-and enables us to observe the time dependence of the wavelength shift, after establishing a magnetic field. The observed wavelength shift was delayed in comparison with the square of the magnetic flux density, which is proportional to the wavelength shift in a steady state. The delay was about 0.07 s, which is much longer than we actually expected, when changing the temperature in an active layer. (C) 1998 Scripta Technica.