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The microfibrils and the microfibrillar network in poly(p-phenylene terephthalamide) (PPTA) fibers formed during the coagulation from a monodomain lyotropic fiber are stabilized by critical point drying and characterized by electron microscopy and small angle X-ray scattering. The diameter of the microfibrils varies from 20 to 49 nm depending on the PPTA concentration in the lyotropic solution used for the spinning. A formation mechanism for the microfibrils and the microfibrillar network is suggested. During the coagulation, the spinodal decomposition (phase separation) is assumed to occur before the crystallization (phase transition), resulting in the formation of the microfibrils. The formation of the microfibrillar network is considered to be related to the misorientation of macromolecules and the density fluctuation in the cross-section of the filament during spinodal decomposition.