The attenuation of longitudinal ultrasonic waves originating from their interaction with phonon gas and from other thermoelastic causes has been evaluated in CsCl, CsBr, and CsI at 300°K by using second‐order elastic moduli and third‐order elastic moduli, in the ⟨100⟩ and ⟨110⟩ directions, and shear waves polarized along the ⟨100⟩ and ⟨11¯0⟩ directions. The phonon viscosity and dislocation drag along the ⟨100⟩ and ⟨110⟩ directions are also discussed.