A motorized device capable of moving in a fluid medium, said device comprising one or more locomotor systems (1; 101), the or each of said locomotor systems comprising a portion referred to as the non-movable portion and a movable portion that is capable of carrying out at least one reciprocating motion with respect to said non-movable portion and is linked to said non-movable portion by means of at least one elastic means (11, 12; 111, 112), said movable portion comprising at least one drive assembly that is linked to at least one motor (4; 104) of said one or more locomotor systems and at least one locomotion member (2; 102) that is linked to at least one drive assembly such that a reciprocating motion of said assembly drives a reciprocating motion of said locomotion member, in this said device: - said at least one elastic means being prestrained, the level of prestrain being sufficient such that the strain on said elastic means does not change sign during said reciprocating motion of said movable portion; - said drive assembly comprises at least one speed reducer (6 and 8; 106 and 108) for reducing the speed of rotation of said at least one motor, said reducer being designed so that when said motor is operating at its maximum mechanical power, the speed of rotation transmitted to said at least one locomotion member is reduced with respect to the speed of rotation of said motor so as to match the speed of rotation conferring a frequency corresponding to said resonant frequency to the reciprocating motion of said at least one locomotion member; said device being characterized in that: - said at least one motor is controlled by a sinusoidal voltage, the frequency of which corresponds to the resonant frequency of said movable portion that is linked to said non-movable portion by said at least one elastic means (2; 102) so that, on the one hand, the frequency of said reciprocating motion of said drive assembly is substantially equal to the resonant frequency of said movable portion that is linked to said non-movable portion by said at least one elastic means and, on the other hand, the instantaneous amplitude of said reciprocating motion of said drive assembly is adjusted so as to control the average position and the maximum amplitude of the reciprocating motion of said at least one locomotion member. - said at least one elastic means being prestrained, the level of prestrain being sufficient such that the strain on said elastic means does not change sign during said reciprocating motion of said movable portion; - said drive assembly comprises at least one speed reducer (6 and 8; 106 and 108) for reducing the speed of rotation of said at least one motor, said reducer being designed so that when said motor is operating at its maximum mechanical power, the speed of rotation transmitted to said at least one locomotion member is reduced with respect to the speed of rotation of said motor so as to match the speed of rotation conferring a frequency corresponding to said resonant frequency to the reciprocating motion of said at least one locomotion member; - said at least one motor is controlled by a sinusoidal voltage, the frequency of which corresponds to the resonant frequency of said movable portion that is linked to said non-movable portion by said at least one elastic means (2; 102) so that, on the one hand, the frequency of said reciprocating motion of said drive assembly is substantially equal to the resonant frequency of said movable portion that is linked to said non-movable portion by said at least one elastic means and, on the other hand, the instantaneous amplitude of said reciprocating motion of said drive assembly is adjusted so as to control the average position and the maximum amplitude of the reciprocating motion of said at least one locomotion member.