A male patient came to our clinic because of a continuous up and down movement of his right testis, which was present even during sleep. The nature and localization of the unilateral rhythmic cremasteric activity is investigated by electrophysiological methods. Investigations of the cremasteric muscles and genitofemoral nerve determined that the movement was similar to myokymia. The localization of this peculiar spontaneous movement is probably in motor fibers somewhere between the L1-L2 motor neuron pool, motor roots and/or the genitofemoral nerve. Published by Elsevier B.V.