The bacteriology and biochemistry of activated sludge grown in domestic waste water or fed with synthetic media were studied. The nature of the flocs was investigated by determining morphological and physiological characteristics of many strains isolated. Predominant bacteria were Achromobacteraceae, further in sludge grown in domestic waste water Pseudomonadaceae and coryneform bacteria, and in sludge grown in waste water from synthetic media Micrococcaceae and large oval encapsulated coccoid bacteria. Bacterial flocs were not formed readily by many of the strains tested in pure culture. Addition of FeCl 3 or CaCl 2 sometimes stimulated flocculation. Variation in C/N relation of the substrate had no effect. Biochemical activities of activated sludge and of some strains in pure culture were studied in Warburg respirometers. Domestic activated sludge readily absorbed organic acids and ethanol, whereas sugars and polyalcohols were taken up slowly. The same held for pure cultures isolated from this sludge. Adaptation to sugar breakdown was-stimulated by N compounds and particularly by amino acids. Dissimilation was 10-20 % for glucose and 30-40 % for acetate. Rate of uptake for glucose and O 2 , and dissimilation percentages were independent of substrate concentration, but proportional to sludge concentration. Almost 60 % of added glucose was stored as polysaccharides by glucose-adapted sludge when no N was supplied. When ammonium sulphate was added only 25 % of the glucose yielded polysaccharides. Endogenous respiration was higher, the higher the sludge load while addition of (NH 4 ) 2 S0 4 with the glucose caused a still higher endogenous respiration, presumably due to amino acid synthesis.