Faecal samples from children between 3 months and 5 yrs were investigated for the presence of methanogenic bacteria. Methanobacteria were not detected in faecal samples obtained from children under 27 months of age. At 27 months only one subject harbored methanobacteria; the number of methanobacteria hosts subsequently increased with age, with an incidence of 40% at 3 years and 60% at 5 years. The appearance of methanobacteria was not directly related to introduction of particular foods in the child's diet, which after 2 years becomes similar to that of Italian adults. These dietary changes could give rise to some physical-chemical modifications of the enteric lumen thus causing the conversion of the intestinal flora to an adult pattern and, in most subjects, the development of methanobacteria.