Bacteria have been implicated in the formation of viscous brown foams that can appear suddenly on wastewater treatment plants. Three strains of the filamentous bacterium Gordonia amarae, isolated from wastewater treatment plants, were investigated to determine their effect on foam formation and stabilisation. During the exponential phase of the bacterial growth a biosurfactant was formed, causing a significant drop in the surface tension of the filtered medium and the formation of persistent foam. Foaming tests in the presence and absence of bacteria showed that bacteria increased foam persistence, most probably by reducing the drainage from the lamellae between bubbles. Experiments showed that > or =55% of the three bacterial strains partitioned into the foam produced by the biosurfactant, indicating that their surfaces were hydrophobic. The extent of partitioning was independent of the growth stage, suggesting that the cell surface hydrophobicity did not change with age, or with cell viability. This work shows that, although the G. amarae cells themselves do not cause foaming, they do produce biosurfactant, which aids foam formation, and they stabilise the foam by reducing the rate of drainage from the foam lamellae.