A study has been made of the influence of various process parameters on the fibrillation characteristics of Lyocell fibers, which are spun from a solution of cellulose in N-methylmorpholine-N-oxide (NMMO). The parameters were air gap length, temperature, and humidity; line speed; draw ratio; polymer solution cellulose and water contents; and coagulation bath concentration and temperature. Fibrillation was induced in the fibers by an ultrasonic treatment and compared by defining a fibrillation index from optical micrographs. By selecting combinations of the parameters described above, fibrillation can either be increased or decreased without significantly affecting the tensile properties of the fibers. A method for spinning nonfibrillable filaments on a laboratory scale is presented.