Abstract The isothermal transformation behaviour of polycrystalline Ni 50 Mn 29 Ga 21 magnetic shape memory alloy is investigated by measuring the saturation magnetisation as a function of time and temperature. The results are analysed within the framework of the Johnson–Mehl–Avrami–formalism. The samples were prepared by directional solidification with a 〈1 0 0〉 texture of the high temperature cubic phase parallel to the heat flow. The samples were heat treated for chemical homogenization and stress relaxation in the austenitic state. They show a thermal hysteresis of martensite to austenite and the reverse transformation. The hysteresis becomes smaller with increasing measurement time. Grain boundary nucleation is a time controlling mechanism during the isothermal transformation in a constant magnetic field in these alloys.