Direct synthesis of rigid-rod aromatic polyamides was studied without condensation reagents by using reaction-induced crystallization of oligomers during solution polymerization. Polymerizations of p-aminobenzoic acid were carried out in an aromatic solvent at a polymerization concentration of 20% at 350 degrees C. High molecular weight poly(p-benzamide) (PBA) was obtained in the form of plate-like crystals, and the highest molecular weight was 11.6 x 10(3). In this polymerization, the oligomers were formed in the solution by the condensation reaction with the elimination of water. When the molecular weight of oligomers exceeded the critical value, the oligomers were precipitated by crystallization via supersaturated state to form the plate-like crystals. The molecular weight increased by the polymerization between the oligomers on and in the crystals. Poly(p-phenylene isophthalamide) (PPIA) was also obtained in the form of plate-like crystals by the polymerization of p-phenylene diamine (PPDA) and isophthalic acid (IPA) under the same condition as that of PBA. These results exhibited the thermally direct synthesis of infusible aromatic polyamides. Sequential addition of monomers into this heterogeneous polymerization was examined to increase the molecular weight. In the step-growth polymerization, the addition of monomers into the homogeneous polymerization could not increase the molecular weight because it was determined by the extent of reaction and reshuffled by the transamidation reaction. However, the addition of PPDA and IPA during the polymerization resulted in the increase in the molecular weight of PPIA. This result gave possibly a novel procedure for the preparation of high molecular weight condensation-type polymers.