Cytolethal distending toxin (CLDT) production by Campylobacter fetus subsp. fetus isolated from calves was examined using CHO cells. Twenty-five of the 26 strains tested were positive for CLDT with titer of ranging 1:8 to 1:2, 048. CLDT positive strains were divided into low and high cytotoxin titer groups. Isolates from diarrhea cases tended to produce a significant ambunt of CLDT compared with isolates from liver. CLDT produced by C. fetus showed no effect on Y-1 cells and was heat-labile and trypsin-sensitive. © 1993, JAPANESE SOCIETY OF VETERINARY SCIENCE. All rights reserved.