Morphometric analysis of cerebellar foliation patterns at the midsagittal plane was done in the inbred strains BALB/cByJ, C57BL/6ByJ and their 7 recombinant inbred strains in order to assess possible major gene influences. The cerebellum was dissected away from the brainstem and weighed prior to histology so that measures of the depth of fissures and sulci could be related to overall size of the brain and cerebellum. Results for 177 mice revealed that many brains had extra sulci present within the central lobe, the culmen, the declive and the uvula, and that patterns within a genetically uniform inbred strain were highly variable. The measure of sulcus depth was continuous, showing no evidence of a normal versus abnormal dichotomy. Furthermore, the frequency and depth of extra sulci were greater in mice with larger cerebella. Strain differences in size of the whole brain and cerebellum clearly resulted from several genetic loci. This was also true of the extra sulci, except for the declival sulcus which revealed a single gene influence. The gene symbol 'declival sulcus of cerebellum' (dsc) is proposed.