1. The numbers of bacteria and fungi in a newly-made Stilton may rise to the enormous number of 1000 to 3000 millions per gram in the first week.2. There is a gradual fall in the numbers up to the time of ripeness (100 to 150 days old), when 50 to 100 millions only are found.3. In the early stages lactic acid bacteria are most abundant. When the cheese is ripe the lactic acid bacteria are few and weakened in physiological power; Penicillium glaucum and a form of Torula are then abundant.4. Five characteristic organisms are found in all Stilton cheeses examined, viz.:—(1) Streptococcus lacticus, (2) a short rod-shaped form of Bs. acidi lactici, (3) a species of Tyrothrix, (4) Penicillium glaucum, and (5) a round form of Torula, sometimes accompanied or replaced by an oval form.In cheeses where starters have been used we find a large celled form of Streptococcus lacticus present.5. Penicillium glaucum is checked in its growth by the Tyrothrix.