A regulation system for controlling the vibratory behavior in twisting and/or the twisting stability of a drivetrain of a rotorcraft, the regulation system comprising two regulation loops that are interleaved one in the other, the two regulation loops being arranged “in cascade” relative to each other to regulate firstly a first speed of rotation NG of a gas generator of at least one turboshaft engine driving said drivetrain in rotation, and secondly a second speed of rotation NTL of a free turbine of the engine(s).