Clostridium difficile, the causative agent of antibiotic-associated pseudomembranous colitis, produces two toxins, A and B (1). Toxin A is both an enterotoxin and cytotoxin, whereas toxin B is a highly potent cytotoxin (2,3,4) . Toxin A has been cloned and sequenced by our research team (5) and later by Sauerborn and von Eichel-Streiber (6). The entire coding region of toxin B and 398 bp of the 5' flanking region were contained in two clones, pCD19 (5) and pCD19L. The sequence is presented below. The open reading frame is 7,098 bp long and codes for 2,366 amino acids. The gene contains 27.4 mol% G + C and a deduced polypeptide molecular weight of 269,696 Da with a pi value of 4.24. These data are in good agreement with molecular characteristics we have previously described for toxin B (7, 8). The 3' end of this sequence is contiguous with the 5' flanking region of the small open reading frame which is located between toxin A and toxin B (5).