A new theory of lower hybrid resonance (LHR) wave excitation by whistlers in magnetospheric plasma is developed. Two time dependent differential equations, which have a form of linear wave interaction between quasi-longitudinal shistler-mode waves and quasi-electrostatic LHR waves in a cold plasma with small-scale density irregularities, are derived and solved by the method of successive approximations. Fundamental observational results on LHR wave excitation by whistlers, such as the lower cutoff frequency of excited waves and the pronounced spectral maximum near the LHR frequency, as well as the close connection of the excitation process with small-scale plasma density irregularities are explained by the present theory in a consistent manner. ¿ American Geophysical Union 1994