The systematic status of a mound-building mouse population from the Adriatic coast at Ulcinj, Montenegro (Yugoslavia), originally reported as M. hortulanus ssp., is assessed using morphological and morphometric analyses. This population was compared to other M. spicilegus samples from Ukraine & Moldavia, Voivodina (Yugoslavia) and Austria, and to M. macedonicus from Turkey, Greece, and Macedonia. Character states of selected non-metric traits, along with the presence of a typical mound-building behaviour, allowed classification of the study population with M. spicilegus. This was also supported by Canonical Variate Analysis on size-adjusted dental traits. Uni- and multivariate morphometric studies revealed marked differentiation between the Ulcinj sample and the remaining M. spicilegus populations. The great deal of morphological divergence suggests a distinct systematic position of the Adriatic population, here described as Mus spicilegus adriaticus ssp. nova. Hypotheses about the possible origin and evolutionary history of the two eastern aboriginal mouse species are discussed.