In order to elucidate the nerve supply of a supernumerary muscle observed between the adductors brevis and minimus in humans and to investigate its true nature and mechanism of formation,100 body halves from 50 adult Japanese cadavers were subjected to gross anatomical examination. A supernumerary muscle was noted in 33 (33.0%) out of the 100 thighs. In each of these thighs, it arose from the upper part of the inferior ramus of the pubis and ran obliquely downwards and laterally. It was inserted into the anterior surface of the insertion aponeurosis of the adductor minimus (17/33 thighs,51.5%), the upper part of the pectineal line (9/33thighs,27.3%), or the posterior side of the base of the lesser trochanter (7/33 thighs,21.2%). It was supplied, from its posterior aspect, by a filament from the twig originating from the posterior branch of the obturator nerve and being distributed to the superficial fasciculus of the obturator externus (18/33 thighs,54.5%) or by a twig directly originating from the posterior branch (15/33 thighs,45.5%). The obturator nerve received fibers from L1234 (4/33 thighs), L234(25/33 thighs), L2345 (2/33 thighs) or L34 (2/33 thighs) and, moreover, its posterior branch ran through (25/33 thighs,75.8%) or over (8/33 thighs,24.3%) the obturator externus to emerge into the thigh. Based on the topographical-anatomical relationships of this muscle to its nerve supply, it seems probable that it is formed by separation from the superficial layer of the obturator externus and changes into an independent structure during the process of ontogeny. Furthermore, from a statistical standpoint, the segmental composition or course of the obturator nerve is not related to either the formation or incidence of this muscle.