Molecular association of methyl iodide, CH3I, is dominated by electrostatic interactions in the crystalline state. Similar interactions govern the molecular association in dihalogen crystals, Cl2, Br2 and I2, and they are the reason of their isostructural relations with the crystals of halomethanes. The prototypic Cmca space group symmetry of the dihalogens reduces to Cmc21 for CH3Cl and to Pnma for CH3Br and CH3I, due to the reduced molecular symmetry and domination of the C–H⋯halogen interactions in CH3Cl and halogen⋯halogen interactions in CH3Br and CH3I. High pressure X-ray diffraction data recorded for CH3I in situ crystallized at 0.53, 1.16 and 2.53 GPa (all at 295 K) confirm its monomorphic structure, orthorhombic space groupPnma, in this pressure range, with the crystals of CH3I at 193 K and CD3I at 4.5 K. The CH3I crystal compression is consistent with the directions of halogen⋯halogen and C–H⋯halogen intermolecular electrostatic interactions in the structure.