The thermal instability of a layer of Rivlin-Ericksen elastico-viscous rotating fluid in a porous medium in hydromagnetics is considered. For stationary convection, the Rivlin-Ericksen elastico-viscous fluid behaves like an ordinary (Newtonian) fluid. The magnetic field is found to have a stabilizing effect on the thermal instability of a layer of Rivlin-Ericksen fluid in the absence of rotation whereas the medium permeability has a destabilizing effect on thermal instability of Rivlin-Ericksen fluid in the absence of rotation. Rotation always has a stabilizing effect. The magnetic field, medium permeability and rotation introduce oscillatory modes in the system, which were non-existent in their absence. The case of over stability is also considered and the sufficient conditions for the non-existence of over stability are obtained in the process. The study finds applications in geophysics, chemical technology and engineering. Among the applications in engineering disciplines one can find the food process industry, chemical process industry, solidification and centrifugal casting of metals and rotating machinery
