Enterotoxigenic Bacteroides fragilis (ETBF) was isolated from intestinal contents of 10 of 45 pigs with diarrhea. Of 10 ETBF-positive pigs, seven were 1 to 4 weeks of age, whereas three pigs had diarrhea after weaning (6 to 8 weeks of age). Of ten pigs infected with ETBF, nine were also infected with K88-positive Escherichia coli, rotavirus, coronavirus, coccidia, or Cryptosporidium. Of 11 conventional pigs exposed to ETBF at 1 to 2 days of age, two subsequently developed fatal postweaning diarrhea. An ETBF isolate, serologically similar to the challenge-exposure strain, was isolated from the feces of the two diarrheal pigs, but not from three healthy littermates. Fatal enteric disease occurred in 11 of 12 adult rabbits with ligated ceca after approximately 5 X 10(9) colony-forming units of ETBF were injected into the ileum. Disease in rabbits was characterized by anorexia and severe, mucoid, hemorrhagic diarrhea. Of 11 rabbits given non-ETBF or sterile brain-heart infusion broth, ten remained clinically normal during the 7-day observation period.