The filamentous fungus Caldariomyces fumago secretes a chloroperoxidase (CPO). To increase its production, we integrated a CPO-expression cassette into the non-transcribed spacer regions of the rDNA in C. fumago. One strain was obtained that had twice the CPO activity when grown in shake-flask and bioreactor compared to the wild-type. The highest CPO activity from the bioreactor cultivation was 3,236 U ml(-1). This is the highest value reported so far.