It is shown by an analysis similar to that for the spinodal decomposition of a supersaturated solution that an array of dislocations, modelled by parallel screw dislocations, of uniform density, is unstable; the dislocations move to form a structure having a modulated dislocation density. It is suggested that the instability grows ultimately into a dislocation cell structure and that the cell size is given by the dominant wavelength of the density modulation. This wavelength &lgr;mis found to be proportional to &rgr;−1/2and furthermore the wavelength is given by &lgr;m≈Kc·&rgr;−1/2=rc, whereKcis a constant, &rgr; is the dislocation density andrcis defined as a dislocation‐dislocation interaction distance. Data in the literature relating to cell size are shown to support this result. Restrictions on the applicability of the analysis are discussed.