Ab initio total energy calculations based on density functional theory and the generalized gradient approximation in conjunction with a constant-pressure minimization algorithm have been used to investigate the pressure-induced structural phase transitions in CsI. According to the calculations, increasing the pressure above causes a small continuous distortion of the ambient pressure B2 structure. At higher pressures, a weakly first-order isosymmetric transition occurs at . The high-pressure polymorph has an orthorhombic `hcp-like' structure with space group Pmma.