SUMMARY: By direct viable counts in deep cultures in a basal nutrient agar, consisting of buffered and very highly clarified nunen liquor, supplemented only with vitamin-free acid hydrolysate of casein, tryptophan, a fermentable carbohydrate and agar, the crude wet nunen contents of a hay-fed sheep has been shown to contain a population of c. lo8 viable and facultatively anaerobic bacteria per g., mostly Gram-positive cocci capable of fermenting'a wide range of soluble carbohydrates including starch and inulin but not mannitol. "he mannitol-fermenting population is of the order of lo6 organisms per g., consisting mostly of Gramnegative rods, including coliform bacteria. Addition of reducing substances such as sodium sulphide or memaptoacetate (thioglycollate) to the basal agar had little or no effect in increasing the counts. An extract of hay made With cleared and sterile rumen liquor did not yield the typical nunen streptococcal flora when counted by this method in rafFinose+rumen liquor nutrient agar.Although most workers with rumen micro-organisms seem to have assumed either that the important fermentative bacteria and protozoa in this very mixed culture must be obligate anaerobes, or that they will not act except under powerful reducing conditions (see, e.g. Hungate (1950) ; Huhtanen, Rogers & Gall (1952) and previous papers by Gall and associates on the methods for isolating nunen bacteria; Sijpesteijn & Elsden (1952) on methods of using washed suspensions of mixed rumen bacteria for fermentation studies), nevertheless there are good reasons for believing that the rumen normally contains a considerable population of facultative anaerobes which are active in fermenting carbohydrates. Thus, by a simple enrichment technique not carried on for more than 18 hr., Heald (1952a, b) isolated numerous pure cultures of coliform bacteria, capable rapidly of fermenting glucose and xylose, from the rumens of hay-fed sheep. MacPherson (1953), in a study of nunen amylolytic bacteria, without any difficulty isolated numerous facultatively anaerobic streptococci, which resembled Strep. bovis in fermentation reactions, from the rumens of both concentrate-fed and hay-fed sheep. These bacteria were absent from the fodder. Sugden & Oxford (1952) showed that an important and ubiquitous group of sheep's rumen ciliate protozoa, the holotrichs, which are very active in fermenting glucose (Heald & Oxford, 1958), are not a t all exacting in their demands for anaerobic conditions and will
