Surface tensions, gamma, of 19 kinds of colloidal spheres of monodispersed polystyrene and silica (6-460 nm in diameter) in crystal-like, liquid-like, and gas-like suspensions are studied systematically at the air-water interface by the Wilhelmy method, gamma-values of the aqueous suspensions of colloidal silica spheres are close to that of pure water, though very weak surface activity [maximum in Delta gamma (surface tension of suspension minus that of water) is ca. -2 mN/m only] is detected for silica spheres of diameters ranging from 100 to 200 nm. The surface activity of polystyrene spheres, on the other hand, is high especially for spheres of diameters between 100 and 200 nm, The maximum in -Delta gamma is ca. 20 mN/m. The large difference in the surface activity between the two kinds of spheres is due to the difference in surface characters, i.e., highly polar and strongly hydrophobic for silica and polystyrene spheres, respectively, Furthermore, surface tension lowering of the crystal-like suspensions is substantial when compared with that of the liquid-like or gas-like suspensions. It is highly plausible that the intersphere distance in the two-dimensional colloidal crystals at the interface is shorter compared with that in the bulk phase by the shortened electrical double layers at the interface.