A fiber of sea-islands phase separation wherein the sea component comprises a vinyl alcohol based polymer with high orientation and great crystallinity and the islands component comprises a water-insoluble cellulose based polymer with excellent absorptivity of alkaline solutions, thermal resistance and heat fusion resistance, and wherein the size of the islands is 0.03 to 10 .mu.m and the strength is 3 g/d or more, is readily disintegrated into a fibril of a diameter of 0.05 to 8 .mu.m when a mechanical stress is imposed onto the fiber wet in water. From the fibril with good hydrophilicity, high strength, great particle captivity and excellent reinforcing performance, and additionally with good absorptivity of alkaline solutions and great thermal resistance and heat fusion resistance, none of the fiber components therein is solubilized during fibrillation. Neither a beating process nor a beating solution causes foaming or environmental pollution. The fibril is extremely useful for use in separator sheets for alkaline batteries, reinforcing fibers of cement slate plates, reinforcing fibers of frictional materials and the like.