  We evaluate quasinormal modes of massive scalar field of the Ernst spacetime,
an exact solution of the Einstein-Maxwell equations describing a black hole
immersed in uniform magnetic field $B$. It is known that the quasinormal
spectrum for massive scalar field in the vicinity of the magnetized black holes
acquires an effective mass $\mu_{eff}= \sqrt{4B^2 m^2+\mu^2}$, where $m$ is the
azimuthal number and $\mu$ is the mass of scalar field. The numerical result
shows that increasing of the field effective mass and the magnetic field $B$
gives rise to decreasing of the imaginary part of the quasinormal modes until
reaching the vanishing damping rate.
