The formation of extensive, biological foams or scums on the liquid surfaces of domestic activated sludge plants in Australia was investigated. A survey of the problem in Queensland and Australia was carried out; the dominant organisms in the foam were isolated and identified; their physiology was investigated and the intrinsic reasons for foam formation were explored in the laboratory. Foaming in activated sludge plants was found to be a significant problem in Australia. Greater than 90% of plants in Queensland have experienced biological foaming episodes, whilst in 38% of the plants the problem was continuously present. Nocardia amarae and a newly described actinomycete Nocardia pinensis were the dominant organisms isolated from the foam. Plants that contained N. pinensis in the scum were found to be operated at long sludge ages. Although technical problems were experienced with chemostat methodology when applied to N. amarae, a limited amount of information about their physiology was obtained. The cells of N. amarae were found to be hydrophobic and a surfactant(s) was produced in culture. These are properties which suggest an explanation for the mechanism of foam formation based upon adsorptive bubble separation theory.