PACS. 47.20.Ma -Hydrodynamic stability and instability. PACS. 75.50.Mm -Magnetic liquids.Abstract. -The dependence of the maximal growth rate of the modes of the Rosensweig instability on the properties of the magnetic fluid and the external magnetic induction is studied. An expansion and a fit procedure are applied in the appropriate ranges of the supercritical inductionB. With increasingB the scaling of the maximal growth rate changes from linear to a combination of linear and square-root-like scaling. The scaling of the corresponding wave number alternates from quadratic to primarily linear. For very smallB the dependence of the maximal growth rate on the viscosity is given. Suggestions are made for experiments to test the predicted scaling behaviours.Introduction. -The investigation of instabilities in magnetic fluids has a long history where the most prominent instability being the normal field or Rosensweig instability [1,2]. Above a threshold of the induction, the initially flat surface exhibits a stationary array of peaks. Despite its long history, some aspects of the Rosensweig instability have been addressed only recently: the hexagon-square transition [3] or the wave number selection problem [3,4]. The wave number is the absolute value of the wave vector, q = |q|, which characterizes small disturbances. The ground state of a pattern forming system is subjected to such small disturbances in order to study its stability. The corresponding growth rate ω may depend on system parameters {P}, e.g. the viscosity of the fluid, and control parameters {R}, e.g. an external magnetic field, ω = ω(q; {P}, {R}). Generally it is assumed that in the linear stage of the pattern forming process the wave number with the largest growth rate will prevail. Therefore this mode is called linearly most unstable mode. Due to its role in the pattern formation it is of particular interest to examine the maximal growth rate ω m , and its dependence on the different parameters.This dependence has received only limited attention in classical hydrodynamic systems. For the Küppers-Lortz (KL) instability in Rayleigh-Bénard convection rotated about a vertical axis, the growth rate of different KL angles were calculated for one fixed rotation rate and different temperature differences [5]. In surface-tension-driven Bénard convection the growth rates are calculated for two fixed values of heat loss and different temperature differences [6]. But in both systems it was not analysed how ω m depends on the control parameters. In the problem of convection for autocatalytic reaction fronts, the maximal growth rate was analysed