This is a multicenter, explorative, phase IIIb, open-label study in patients with CF and PI. The study consists of a screening visit (visit 1), followed by a baseline phase of 9 days (plus a 5-day window if necessary) during which the regular pancreatic enzyme will be maintained and 10 stool samples will be collected over 5 days, for baseline evaluation of steatorrhea. Afterward, a treatment phase of 19 days (plus a 5-day window if necessary) with Ultrase® MT12 will follow (the usual pancreatic enzyme will be replaced by Ultrase® MT12). Over the last 5 days of the treatment phase, 10 additional stool samples will be collected, for evaluation of steatorrhea.