Flotation harvesting of cultured Chlorella was investigated by examining the surface charge of microalgae and the foaming ability of culture solution. With decreasing pH of the culture solution, microalgae Chlorella were more effectively recovered, probably due to the decrease in electrostatic repulsion between gas bubbles and microalgal cells. The cell concentration in the solution decreased exponentially with time, and the separation rate constant increased with the decrease in bubble diameter and the increase in gas holdup, The amount of recovered microalgae increased with the gas flow rate, and it was shown that nearly perfect recovery was attainable by adjusting the operational conditions. The microalgae were concentrated in the froth layer formed above the surface of the liquid column, and the layer became more deeper and stable with decreasing solution pH. The cell concentration in the recovered solution increased with the depth of the froth layer, and it was possible to achieve high enrichment of 20 to 40 times the cell concentration of the culture solution. Capillary electrophoresis of metabolites produced from the microalgae showed that the charge of metabolites approached zero with decreasing solution pH. This suggests that the metabolites contribute to the decrease in electrostatic repulsion and the stability of the froth layer.