The influence of dietary ferric iron on the intestinal microbiota of mice was investigated with a view to promoting benign lactic acid bacteria (which have minimal iron requirements) in order to enhance colonization-resistance potential. Three groups of eight mice received a diet differing only in iron content, for a period of 12 weeks. Dietary iron deprivation resulted in overall increased small intestinal bacterial populations, including lactic acid bacteria, but these differences were generally not significant (p > 0.05). With the exception of coliforms, all examined bacterial groups (anaerobes, micro-aerophiles, lactobacilli, and enterococci) were significantly (p < 0.05) elevated in the colons of iron-deprived mice. The relatively low numbers of total anaerobes in the colons of iron-replete and iron-overloaded mice suggested that, as well as promotion of bacteria under iron-deprived condition, provision of ferric iron suppressed bacteria, probably by oxidation of normally reduced environments.