The organism to be described in this paper was isolated in the spring of 1938 from a bottle of domestic red wine turning sour. From wine souring to vinegar, one ordinarily expects to isolate bacteria belonging to the genus Acetobacter; however, careful examination both microscopically and by plating on nutrient agar failed to reveal members of this genus as present. On the contrary, a yeast was found in large numbers. So far as could be determined, the yeast was the sole occupant of the liquid in the bottle, thus confronting us with that very rare condition, namely, a pure culture in nature. This condition may have resulted, in part, from the high acidity of the wine for the pH was determined colorimetrically to be 2.2. Single cell isolations of the organism were made using th'e Hansen technique; several isolates kept in culture for over a year showed the same properties and showed no variation in that time. The isolate kept in culture for the past four years, similarly, has shown no sectoring nor other variation. The first attempts at identification of this yeast met with considerable difficulty. I tentatively considered the organism as belonging to the genus Mycoderma which includes imperfect yeasts, widespread in air, and frequently to be found in alcoholic solutions. They ordinarily form a scum on the surface of liquid media and rarely carry out an alcoholic fermentation, but are notorious for the production of acid from alcohol. Mycoderma vini (according to Lodder's 1934 classification) was the closest approach that could be made for this isolate but the position was actually untenable since no pellicle was ever formed by the yeast on any liquid medium and since the yeast did carry out an alcoholic fermentation.