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It has been proposed that single crystals of phenothiazine undergo a proper ferroelastic phase transition: Pnma(D2h)→P21/c (C2h). This premise is in conflict with Brillouin-scattering studies from both quasi- and high-symmetry longitudinal and transverse acoustic modes reported here. The C55 elastic constant of phenothiazine as an orthorhombic crystal at ambient conditions shows no temperature dependence. The inelastic, light-scattering spectra support an order-disorder phase transition; no significantly unstable acoustic phonons were observed through the phase transition. Laser diffraction from structural domains reveals that the crystal is polysynthetically twinned above and below Tc. At room temperature, domain walls are made indistinct by disorder but become visible as the sample is cooled. Stress-strain hysteresis below Tc is attributed to a ferrobielastic effect.