An outbreak of diarrhea spanning 3 months occurred in a breeding colony of scid/Trp53 knockout mice. Approximately a third of the 150 mice were clinically affected, with signs ranging from mucoid or watery diarrhea to severe hemorrhagic diarrhea with mortality. Helicobacter bilis and the newly recognized urease-negative organism H. rodentium were isolated from microaerobic culture of feces or cecal specimens from affected mice. Dual infection with H. bilis and H. rodentium were confirmed by culture and polymerase chain reaction (PCR) in several animals. Both Helicobacter species rapidly colonized immunocompetent sentinel mice exposed to bedding from cages containing affected mice, but the sentinel remained asymptomatic. Mice with diarrhea had multifocal to segmental proliferative typhlitis, colitis, and proctitis. Several affected mice had multifocal mucosal necrosis with a few focal ulcers in the cecum, colon, and rectum. Mice with diarrhea were treated with antibiotic food wafers (1.5 mg of amoxicillin, 0.69 mg of metronidazole, and 0.185 mg of bismuth/mouse per day) previously shown to eradicate H. hepaticus in immunocompetent mice. Antibiotic treatment resulted in resolution of diarrhea, but not eradication of H. bilis and H. rodentium; mice continued to have positive PCR results after a 2-week treatment regimen, and clinical signs of diarrhea returned in some mice when treatment was suspended. To the authors' knowledge, this is the first report of natural infection with either H. bilis and/or H. rodentium causing acute diarrheal disease and suggests that H. bilis and/or H. rodentium can be an important pathogen for scid mice.