The segmental dynamics of the p-phenylenediamine rings of poly(p-phenyleneterephthalamide) (PPTA) have been examined via two-dimensional exchange solid state H-2 n.m.r. spectroscopy. Fully and partially relaxed experiments were performed over the temperature range -119 degrees C to 75 degrees C. Two populations of phenylene rings can be differentiated by their spin-lattice relaxation times and are associated with sites at the crystallite surfaces and the crystallite interiors; each population is found to be quite heterogeneous over the temperature range examined.