Air was bubbled through sea water in an all-glass 360 ml foam tower. Bacterial counts, fluorescence and ultra-violet absorption were determined in the condensed foam fraction collected at intervals over a 4-hour period. The natural population and pure cultures of bacteria were concentrated from filtered and unfiltered sea-water samples. Concentration factors (the ratio of cells per ml in collected foam to cells per ml in the tower sea water prior to foam collection) were 3–16 for 0·5 hr bubbling time in filtered sea water containing pure cultures of bacteria. Unfiltered sea-water samples, containing particulate organic detritis, gave concentration factors of 16 and 33 in 0·5 hr bubbling time for a pure culture of bacteria and the indigenous population, respectively. The bacterial morphology influenced the extent of concentration and there was evidence of some lysis of cells in the collected foam.