Summary: The results of tests of a circular fuselage with various combinations of tail lengths and vertical tail surfaces with and without the horizontal tail surface in the 6- by 6-foot test section of the NACA stability tunnel are reported in the form of diagrams of variation of coefficients of lateral force and yawing moment with angle of yaw and angle of attack. The results of these tests indicated that the change in the unstable yawing moment of the fuselage alone due to increased tail length did not appreciably affect the yawing moment of a fuselage and vertical-tail combination. The addition of a horizontal tail increased the efficiency of the vertical tail in normal-flight attitudes and in the region of negative angles of attack. Existing methods of computing tail effectiveness gave results within ±7 percent of the measured values for the cases computed.