A hollow fiber of cuprammonium regenerated cellulose having an axially disposed cylindrical bore extending throughout the fiber length, said bore being filled with a gas. The hollow fiber exhibits an improved dialysis performance and ultrafiltration rate over the conventional hollow fiber. Such a hollow fiber can be produced by a process comprising (a) extruding a spinning dope through an annular orifice while simultaneously injecting a gas into the bore as it is formed, and (b) allowing the fiber extrudate to free fall in an air space and dive into a depth of from 2 to 20 mm below the surface of a coagulation bath only with a downward force gained during said free fall.