Poly(4-mercaptobenzoyl) (PMB) whiskers were obtained from S-acetyl-4-mercaptobenzoic acid by high-temperature solution polymerization. The whisker prepared in liquid paraffin at a concentration of 1.0% at 300-degrees-C has an almost uniform width of 0.3-0.5 mum and a length of 5-8 mum. These whiskers show radial growth from the centre. The optimal conditions for preparing them are by using liquid paraffin as a solvent and polymerization temperatures of 260-300-degrees-C. Electron diffraction reveals that the PMB whiskers show single crystal nature and the polymer chains are aligned along the long axis of the whisker. D.s.c. analysis of the PMB whiskers shows the presence of the reversible solid-solid transition as already reported by Kricheldorf et al. The transition temperature and enthalpy of the PMB whiskers are higher and larger than those of PMB crystals with other morphologies. Another endotherm is found at approximately 420-degrees-C, but only when measurement is carried out using a faster heating rate of 100-degrees-C min-1. This is deduced to be a solid-liquid crystal transition from the observation of the nematic domain structure in a quenched specimen using an optical microscope. Finally, the formation mechanism of the PMB whiskers is discussed from the polymerization time dependencies of the length, the degree of polymerization and the yield.