1. The lactic acid flora of Cheddar cheese made from milk of certified quality form a well-defined, physiologically homogeneous group of bacteria, growing best over a temperature range of from 22 to 37° C. They may be classified into four well-defined types, Str. lactis, Str. cremoris, Sbm. plantarum and Sbm. casei, and have been studied over a period of five years. It appears from the evidence found that Str. lactis and Str. cremoris are distinct species, but that Sbm. casei and Sbm. plantarum represent different stages in the adaptation of a common progenitor to conditions in a ripening cheese. Both the streptococci and the streptobacteria appear to be unable to oxidise sugars and may thus be considered indifferent to molecular oxygen.

2. A study of their frequency distribution from the curd at making to an 18 months old cheese has shown that Str. lactis and Str. cremoris are equally viable during the first month, after which the rod forms begin to predominate, Sbm. plantarum and, later, Sbm. casei being found. The former lactobacillus is only found when the cheese is from 1 to 5 months old, the flora consisting entirely of Sbm. casei after this time. The general vigour of all strains decreases with increasing age of the cheese. There is a marked correlation between the shape of the cell, the viability of the organism in cheese and its resistance to acids and lactates.

3. The factors controlling the sequence of flora in Cheddar cheese are discussed. There is no evidence that titratable acidity, oxygen tension and differential carbon sources are responsible for the sequence. It is suggested that lactate concentration, the extent of protein degradation and osmotic pressure are factors responsible for the gradual replacement of the streptococci by the rod forms.

4. The significance of sugar fermentations by the lactic 'acid bacteria studied is discussed. The slow production of lactase is shown to be the reason for the slow growth of weakened strains in litmus milk.

5. Str. cremoris predominates over Str. lactis in the depth of the cheese in the early stages of ripening, whereas near the surface the reverse holds. Certain strains of Str. cremoris isolated from the depth of the cheese were particularly vigorous in growth in litmus milk, forming gas and beginning to peptonise the milk in about 3 days. Such strains consisted of very long chains of large cells of peculiar morphology. It is suggested that this finding is related to the known greater rate of ripening in the depth of the cheese.