The influences of temperature, concentration of swelling agents and fiber materials on the fibrillation tendency in various cellulosic fibers in aqueous solutions were investigated in terms of fibrillation stability and fibrillation sensitivities to alkali and heat. The fibrillation stability and the fibrillation sensitivity to swelling agents were evaluated with a critical point of fibrillation (CPFconc.) that is the concentration of the swelling agents where fibrillation begins, and the ratio of initial increase in fibril number to increase in concentration of swelling agent (Ii). The fibrillation sensitivity to heat was estimated with the increase in Ii against temperature. The CPFconc. of lyocell fiber was 16.7 mol/l water in ethanol/water mixture at 25 °C and decreased to 0 mol/l at 80 °C, indicating acceleration of the fibrillation at higher temperatures. The Ii of lyocell was enhanced from 3.50 to 7.57 count l/mol. The CPFconc. increased in the order of viscose > cross-linked lyocell > modal > lyocell while the Ii decreased in the order of viscose < modal < cross-linked lyocell < lyocell at 40 °C. The Ii of lyocell fiber increased to the greatest extent with increase in temperature as compared with the other cellulosic fibers. Lyocell fiber has the lowest fibrillation stability and the highest fibrillation sensitivities to alkali and to heat resulting in the highest fibrillation tendency. © Springer 2005.