A Simple Tandem Repeat sequence of 11 nucleotides has been found in the ITS1 region of the rDNA of members of Order Xylariales. The number of repetitions detected ranged from one to six, and they could be found in pure tandem or interspersed. The same core sequences have also been found in DNA from other organisms, although usually not repeated in tandem. These repetitions could have been generated by slipped strand mispairing. The presence of this sequence increases the normal rate of divergence in the ITS1 of the Xylariales. The phylogenetic implications of the presence of this sequence in the molecular taxonomy of Xylariales are also discussed.