A slightly improved version of the continuous flow diffusion cloud chamber which has been described earlier (Hudson and Squires, 1973) has been rotated so that the plates are vertical instead of horizontal, the air flow remaining horizontal. Without other modification, its count agrees with that of the improved horizontal version to within about 1% over the supersaturation range 1.0–0.5%. The vertical mode of operation has certain advantages over even the improved horizontal chamber; in particular, it extends downward the supersaturation range over which reliable results can be obtained. Satisfactory plateaus can be obtained at least down to a supersaturation of about 0.1%. Two vertical chambers have been found to agree to about 1% in the supersaturation range 1.0–0.25%, and to about 10% down to a supersaturation of 0.1%.