The nutation of plant organs has been considered to be either the result of a geotropic feedback loop or produced by an endogenous oscillator. Observations have been made of the angular displacement of Phaseolus seedlings during response to a gravitational stimulus. Nutational oscillations have been observed during the corrective movement of the stimulated plants in many cases. Geotropic movement can, however, occur in plants not exhibiting nutation. These observations are considered to support the hypothesis that the movements of nutation in Phaseolus have an endogenous origin unconnected with geotropic reactions. A theoretical model is proposed to account for the observed nutational behaviour of bean seedlings.