A therapeutic method of treating diarrhea of a patient, such as that caused by rotavirus in which a liquid suspension of one or more strains of Lactobacillus reuteni is administered to the patient. Preferably the L. reuteri is isolated from an animal of the same species as the animal to which the therapy is being given. Preferably at least about 10.sup.7 cells of L. reuteri, and most preferably, at least 10.sup.8 cells, are administrated per day, over a period of one to seven days, depending on the severity of the gastroenteritis. The result is a rapid, dramatic reduction in animal's diarrhea and vomiting, previously not found using other therapies.