The crystal structure of the C form of stearic acid has been determined from X-ray powder diffraction patterns, The crystals are monoclinic, space group P21/a, with unit-cell dimensions: a= 9·36, b= 4·95, c= 50·7 Å. β= 128 °15′ and Z= 4. The crystal structure has been determined by trial-and-error methods to R 0·086. The structural parameters are compared with those of similar compounds, in particular the same acid in the polymorphous B form.