Keeping in view the increased usage of orthorhombic materials in the development of advanced engineering materials such as fibers and composites and other multilayered media, the aim of the present paper is to give a detailed account of the plane harmonic generalized thermoelastic waves in orthorhombic materials. According to the frequency equation, the four waves, a quasi-longitudinal, two quasi-transverse, and a quasi-thermal wave, can propagate in an orthorhombic crystal. When plane waves propagate along the axis of an orthorhombic solid, then only the longitudinal and thermal waves are coupled, whereas the transverse waves get decoupled from the rest of the motion. For plane waves propagating in one plane of the solid, only the SH wave in that plane remains purely transverse and gets decoupled from the rest of the motion and vice versa. The other three waves are coupled and get modified due to thermal variations and relaxation time. The particle paths and stability of the waves have been discussed and the results verified numerically. These have been represented graphically for single crystals of solid helium and cobalt material. © 2000 Acoustical Society of America.