Intestinal microflora did not play a role in the intensity or course of EDIM rotavirus-induced diarrhea, since similar results were observed in axenic and conventional mice. In conventional mice, rotavirus-induced diarrhea did not modify the establishment of Lactobacillus spp. and Escherichia coli before weaning. The consequences of diarrhea on the establishment of strictly anaerobic bacteria after weaning were studied through the measurement of two bacterial functions, the microbial barrier effect against E. coli and the development of the immunoglobulin A intestinal immune system. These two bacterial functions were expressed in a similar way in diarrheic and control mice. In young gnotobiotic mice inoculated with Clostridium perfringens or C. difficile, rotavirus infection led to an earlier development of both strains, as compared with controls. This effect was more pronounced with C. difficile. These results suggest that rotavirus infections might enhance opportunistic bacterial infections.