Using a twin-engine turboprop aircraft, flight measurements were made to supplement the very limited data on the effects of propellers on the individual lateral static and dynamic stability derivatives. It is found that the propeller slipstream significantly affects the wing contribution to the derivative lv. A significant reduction in recovery time from the spiral mode instability in low-speed, high-power conditions is consequently found. Propeller disk contribution to the overall damping in yaw derivative nr is also found to be significant. Propeller contributions to yv and nv, due to the known "fin" effect, are found to be small for the test aircraft. Existing expressions to calculate propeller influence on lv, nv, and yv are shown to be adequate, while an expression to calculate the propeller contribution to nr, which appears not to have been analyzed before, is presented.