A bacteriological study was conducted on six activated sludge samples from actual plants treating carbohydrate wastes, and the results were compared with the metabolic activities of the sludges and the qualities of the wastes. The sludges tested contain about 10 11 viable bacterial cells per gram dry sludge. About 50 strains of bacteria isolated from each sludge sample were classified into groups according to their morphological and physiological properties. Many isolates could not grow on glucose as the sole source of carbon and energy, even if they were isolated from the sludges treating the wastes which contained predominantly carbohydrate. Most of the dominant groups of the sludges treating the wastes with a low proportion of carbohydrate were Gram-negative rods and were similar to the genera of the sludge grown on sewage reported by many investigators. Their rates of oxygen consumption were higher when they were fed with casamino acids than with glucose. On the other hand, in the sludges treating the wastes with a high proportion of carbohydrate, Gram-positive, non-spore-forming bacteria, probably belonging to Corynebacterium, and lactic acid bacteria and relatively large coccus predominated. Many strains of these groups had the ability to accumulate reserve polysaccharide and showed a higher oxygen consumption with glucose than with casamino acids or acetate.