Poly(4'-oxy-4-biphenylcarbonyl) (POBP) crystals are obtained from 4'-acetoxybiphenyl-4-carboxylic acid by high-temperature solution polycondensation. When the polymerization is carried out in aromatic solvents such as Therm S 800 and Therm S 900, bundlelike aggregates of fibrillar crystals are obtained. Their length and width are 5-10 and 0.1-0.2 mu m. These fibrillar crystals are linked laterally to one another at the central part. The electron diffraction experiments reveal that the polymer chains align along the long axis of the fibrillar crystals. From terphenyl are obtained hexagonal slablike crystals, of which the thickness is 2-3 mu m and the width is on the average 10 mu m. These slablike crystals are also fibrillated in the direction of the thickness. The formation mechanism of the fibrillar crystals was followed by the observation of the crystal morphology and the polymerization time dependencies of the yield of the crystals, the crystal sizes, and the degree of polymerization (DP). Scanning electron microscopic observation of incipient crystals obtained after 1 h showed convex-lens-like shape. Transmission electron microscopic observation of them showed the lamellar steps due to spiral growth at their thin edge. The morphological observation of the crystals obtained after 2 h showed lenslike shape but many protrusions with a width of 0.1-0.2 mu m in their surfaces. This result indicates that the reorganization of crystals is accompanied by an increase of the DP by postpolymerization. From these experimental facts, it can be deduced that the formation mechanism of the bundlelike aggregates of fibrillar crystals is fundamentally similar to that in poly(oxy-1,4-benzenediylcarbonyl) whiskers and the apparent difference in crystal morphology is caused by the reorganization of crystals during slow lamellar crystallization of oligomers formed in solution polymerization.