The coexistence of two phases in concentrated dispersions of monodispersed platelike particles is reported. Small-angle neutron diffraction has been used to identify a highly ordered phase which coexists at equilibrium with a less ordered, less dense phase. The interparticle interactions were Varied by adjusting the salt concentration, and this influenced the concentration at which the phase transition occurred. Columnar or layer structures which could account for the observed diffraction pattern from the ordered phase are discussed.