Some observations on the caecal microflora of the chick during the first two weeks of life.

1. Changes in the caecal flora of chicks aged from approximately 3 h to 14 d were studied. 2. Counts obtained by means of a conventional anaerobic plating method and a method for the stricter anaerobes remained comparable throughout but occasionally it was found that strains of anaerobic bacteria isolated from the predominant flora of 4- to 14-d-old birds could not be grown when tested by the conventional method. 3. During the 14-d period approximately 100% of the organisms isolated on anaerobic plates utilised uric acid up to the third day but the incidence subsequently declined and in one case represented only 4% of the total flora. 4. The predominant uric acid-utilising bacteria were Streptococcus faecalis and Escherichia coli up to 3 d of age and various types of anaerobic bacteria thereafter. 5. None of the isolates showed an absolute requirement for uric acid.