Inbred mice of the DBA/2J and C57BL/6J strains are known to differ in physiological and behavioral characteristics that are partially controlled by nuclei in the preoptic area/anterior hypothalamus. We describe a distinguishing nucleus of darkly staining, densely packed cells, which we term the medioventral pars compacta (MVPC), within the medial preoptic nucleus of DBA/2J, but not C57BL/6J mice. The analysis also indicates that this nucleus is nearly 80% larger in volume in females vs males of the DBA/2J strain. The strain difference may be used to define genetic influences on this neuroanatomical and functional property.