[en] The kinetics of the isothermal martensitic transformation in Fe--29.6 Ni and Fe--22.5 Ni--4 Mn alloys were investigated as a function of constant magnetic fields, ranging up to 140 KOe (11 x 10⁶ ampere/m). The principal role of the applied magnetic field is to increase the free-energy difference between the product and parent phases in these iron-based alloys, and so the transformational driving force for a given composition can be varied even at a fixed temperature. In the Fe--Ni alloy, the isothermal transformation rate is increased by raising the magnetic field as well as by lowering the temperature, leading in either case to the entre of burst characteristics. The Fe--Ni--Mn alloy does not transform perceptibly without an applied field, but exhibits isothermal C-curve kinetics in fields above 60 KOe (4.8 x 10⁶ ampere/m). The average volume per martensitic plate ad the average plate orientation are found to be insensitive to the strength and direction of the magnetic field, undoubtedly indicating the dominance of autocatalytic nucleation during the course of the isothermal transformation. However, there is some tendency for the morphology to become less lath-like and more plate-like with increasing field strength at a given transformation temperature, and likewise with increasing temperature at a given driving force. Evidently, the prevailing view that martensite becomes more plate-like with decreasing transformation temperature is a consequence of the attendant increase in driving force rather than a pure thermal effect. Nucleation rate measurements have also been carried out as a function of temperature and magnetic-field strength. In testing nucleation models against these results, it is necessary to take into account the effects of magnetic field, as well as of temperature, on the elastic constants