Sludges from three full-scale sewage treatment works using the activated sludge process were analysed and the results examined in relation to the formation of stable foam. One of the works had no history of foam formation, and was regarded as a control, the other two did produce foam. The survey showed that the sludge from the control plant had a bacterial profile similar to that of one of the foaming plants and that the two sludges which produced foam had significantly different bacterial profiles. The physico-chemical characteristics of the control sludge were shown to be different from the two sludges which foamed. One of the main differences was the uronic acid content of the extracellular polymers.