The influence of allozyme heterozygosity on developmental homeostasis as indicated by fluctuating morphological asymmetry (FA) has been a controversial issue in evolutionary studies. In the present investigation, relationships between overall individual heterozygosity (H), calculated over 13 polymorphic enzyme loci, and fluctuating asymmetry (FA) in 27 non-metric and 9 metric bilateral skull and mandible traits were examined in a total of 417 brown hares. The respective tests were performed separately for juveniles and adults within and among five geographic arrays of samples, and among 17 single sampling localities. Within geographic units, neither in metric nor in non-metric characters could a clear relationship between FA and H be detected. Among geographic units and single sampling localities, for metric traits the result remained the same. In non-metric traits, however, a significant negative correlation between overall FA and H became apparent in adults. Thus, fluctuating asymmetry and heterozygosity are inversely related also in a mammalian species. The long lasting dispute as to the existence of a homeotherm-poikilotherm dichotomy may have been the result of a disregard of non-metric traits and of an interpopulation approach for assessing relationships between morphological FA and H in mammals.