We analyse the coupled modes propagation in a parallel-plate gyroelectromagnetic biaxial chirowaveguide. The transverse components of electric and magnetic fields in such bianisotropic waveguide are expressed in terms of the longitudinal components ofEandH, and the effects of different constitutive parameters on the dispersion characteristics as well as cut-off frequencies of propagating modes are demonstrated numerically. The results show that the processes of mode bifurcation and conversion also take place in the parallel-plate gyroelectromagnetic biaxial chirowaveguide.