The bacteria causing foaming in activated sludge plants are considered to be hydrophobic, and their hydrophobicity is assumed to be a crucial factor in their foam-forming ability. This study showed no consistent relationship between cell surface hydrophobicity (CSH), as determined by microbial adherence to hydrocarbons, of three Rhodococcus spp. isolated from activated sludge foam and their ability to produce a stable foam. There also appeared to be no correlation between the mycolic acid composition of these strains, in terms of chain length or degree of unsaturation, and either CSH or foaming ability. Zeolite and bentonite successfully prevented foaming by a Rhodococcus sp. in pure culture, which suggests that cell surface charge may also play a role in foam stabilisation.