SUMMARY: Osmophilic yeasts are found in the film of molasses on the crystals of raw sugars and in intermediate sugar-refining products of a wide range of concen- tration. They are allied to yeasts found in similar situations such as concentrated fruit juices, etc., and can be made to spore more or less freely under dry conditions, usually with conjugation. They are unable to destroy sucrose in strongly buffered solutions; some are able to produce sufficient acid from traces of invert sugar present gradually to invert the sucrose. During this process multiplication takes place. Previous workers have shown that the majority of organisms of this group is highly acid resistant. Organisms which grow in a wide range of concentration of dissolved solids (salts, sugar) show, at acid pH values, an adaptation to growth in higher concentrations accompanied by a diminished metabolic rate. This is lost after a period of 6-8 weeks in dilute media, but can be revived in many cases by increasing the sugar concentration in not less than two steps. Temperature resistance is also increased in concentrated media. According to Henrici's monograph (1941) the term osmophilic was first applied by Richter in 1912 to micro-organisms which multiply in solutions of high osmotic pressure. Apart from the brine yeasts, this term is most fre- quently applied to the yeasts tolerant of high sugar concentration to be found in a wide range of natural and manufactured products. Nussbaumer (1910) was the first to find that osmophilic yeasts were a factor in honey spoilage, and Lochhead & Heron (1929) traced these to the honey of various flowers. Kroemer & Krumbholtz (1931) studied similar organisms in fermenting wine musts produced from grapes infected with Botrytis cinerea; Baker & Mrak (1938) found organisms from the same group on crystallized fruit; Sacchetti (1932, 1939) studied the microflora of fruit juices, Italian wine musts and honey; Owen (1948) has reported the presence of Torulopsis spp. in sugars and molasses. More detailed diagnostic work on honey by Lochhead & Farrell (1931) and Lochhead & McMasters (1931), on honey and wine musts by Sacchetti (1932, 1939), and on fruit juices by Ingram (1949a, b) showed that the majority of organisms involved belonged to the sub-genus Zygosaccharomyces Barker and many were osmophilic strains of known species, Kroemer & Krumbholtz (1931) divide the osmophilic zygosaccharomyces on physiological grounds into two groups :