The parasagittal parcellation of the mammalian cerebellar cortex has been revealed using anatomical, electrophysiological, histological and immunological techniques. Correlation studies have been carried out to determine whether a common organizational plan encompassing the various afferent, efferent and intrinsic maps may exist in the mammalian cerebellum. Many of these studies utilized the parasagittal Purkinje cell antigenic banding pattern as revealed by the monoclonal antibody against zebrin antigens as a standard reference. In this study, the pattern of labelled mossy fiber terminals originating from the external cuneate nucleus was determined and compared with the Purkinje cell antigenic zebrin bands in the same sections. External cuneocerebellar fibers in the mouse were observed to project in well-delineated parasagittal terminal distribution zones, primarily to the ipsilateral vermal cerebellar cortex. However, there was a very minor, but consistent contralateral component. The external cuneocerebellar fiber termination pattern in the mouse differed from that seen in other rodents such as the rat. Comparison of the external cuneucerebellar terminal zones in sections immunohistochemically stained for the zebrin II antigen revealed that the boundaries of the terminal fields of the external cuneocerebellar projection do not always align with those of the zebrin II antigenic bands. These results strongly suggest that mossy fibers have a complicated relationship to zebrin defined compartments. Therefore, the designation 'functional unit' of the cerebellum or 'cerebellar module' remains uncertain at this time.