The ecological effect of cefixime on aerobic fecal flora was evaluated in 6 children aged 2 to 5 years given oral cefixime (8 mg/kg/day in two divided doses) for five days. Serial dilutions of stools recovered at baseline (D0), 2 and 5 days after initiation of therapy (D2 and D5) and two days after completion of therapy (DC + 2) were cultured on selective media. Colonies of Enterobacteriaceae, group D streptococci, staphylococci, and Candida were counted. A moderate but significant (p less than 0.01) decrease in the Enterobacteriaceae count was seen, with a fall from 8 log 10 (4.8 to 9.4) on D0 to 6.4 (3.6-9) on D2, 6 (4.6-7.2) on D5 and 4.7 (2-8.2) on DC + 2. No cefixime-resistant Entrobacteriaceae or Pseudomonas strains were evidenced. A slight, non significant decrease in group D streptococci counts was found, with values of 7.1, 7.1, 5.8, and 8.3 log 10 at the successive time points. All children exhibited fecal staphylococcal strains which were all coagulase-negative and which did not undergo noticeable quantitative changes (mean successive values were 2.5, 1.8, 3.5, and 3.8 log 10). Candida were found in four children and also changed little over time (mean successive values: 2, 1.7, 2, and 2.6 log 10). In sum, oral administration of cefixime was associated with a modest decrease in the number of Enterobacteriaceae, with no development of resistance to cefixime.