The hypothesis that motor imagery and actual movement involve overlapping neural structures in the central nervous system is supported by multiple lines of evidence. The aim of this study was to examine the modulation of corticomotor excitability during two types of strategies for motor imagery: Kinesthetic Motor Imagery (KMI) and Visual Motor Imagery (VMI) in a phasic thumb movement task. Transcranial magnetic stimulation (TMS) was applied over the contralateral motor cortex (M1) to elicit motor evoked potentials (MEPs) in the dominant abductor pollicis brevis (APB) and abductor digiti minimi (ADM). In a separate experiment, transcutaneous electrical stimuli were delivered to the median nerve at the dominant wrist, to elicit F-waves from APB. Imagined task performance was paced with a 1 Hz auditory metronome, and stimuli were delivered either 50 ms before (ON phase), or 450 ms after (OFF phase), the metronome beeps. Recordings were also made during two control conditions: Rest, and a Visual Static Imagery (VSI) condition. Significant MEP amplitude facilitation occurred only in APB, and only during the ON phase of KMI. F-wave persistence and amplitude were unaffected by imagery. These results demonstrate that kinesthetic, but not visual, motor imagery modulates corticomotor excitability, primarily at the supraspinal level. These findings have implications for the definition of motor imagery, and for its therapeutic applications.