The premartensitic phase transformation in Ni49.0Mn23.5Ga27.5 (wt pct) has been established by means of neutron powder diffraction and measurements of elastic constants of C44 and C´. The commensurate, modulated premartensitic phase has been verified by stiffening of C44 prior to the martensitic phase transformation. The slope change of C´ at the Curie temperature positively confirms that the precursor phenomena in NiMnGa are enhanced by the magnetoelastic coupling. Magnetically, Ni49.0Mn23.5Ga27.5 premartensite is characterized by the coexistence of a finite dc magnetic susceptibility and a vanishing magnetocrystalline anisotropy, which distinguishes bcc NiMnGa from the typical magnetic soft materials. This property arises from the competition between the exchange forces of the host lattice and the strong local crystal fields stemming from the tweed.