It is widely accepted that intestinal anaerobic bacteria do not translocate from the intestinal tract as readily as facultative species. However, to date this hypothesis has not been tested in defined flora animals where the intestinal concentrations of different bacterial species can be closely monitored. Seven different groups of defined flora mice were colonised with from two to seven different species of bacteria representing various mixtures of intestinal facultative and/ or anaerobic species, including Escherichia coli C25 (streptomycin resistant), E. coli M14 (streptomycin sensitive), Proteus mirabilis, enterococcus, Bacteroides vulgatus, B. fragilis, and Clostridium sp. With the exception of the Clostridium sp., each of these species colonised the caeca of defined flora mice at concentrations ≥ 109 per gram. Each of the four facultative species (i.e., E. coli C25, E. coli M14, P. mirabilis, and enterococcus) translocated to the mesenteric lymph nodes of approximately one-half of the defined flora mice associated with each species (n ≥ 42). In contrast, of the remaining three anaerobic species, only B. vulgatus translocated in only three mice (n = 52), despite caecal population levels of each of the Bacteroides species that were comparable to each of the facultative species. These results indicate that, in animals with a limited intestinal flora, intestinal anaerobic bacteria do not translocate as readily as facultative species and that the mechanism responsible for the translocation of facultative bacteria in preference to anaerobic bacteria is independent of the concentration of intestinal bacteria.