It is widely accepted that the cerebellar cortex of the cat (and other mammals) includes a D1 and a D2 zone. Their climbing fibres originate from the principal subnucleus of the inferior olive (PO) and their Purkinje cells project to the lateral (dentate) cerebellar nucleus (NL). However, the precise topographical relationship between the two lamellae of PO and the two D zones remains unclear. Uncertainty also exists concerning the cortico-nuclear projections. In view of the conflicting evidence and because of our interest in the involvement of the cerebellar hemispheres in the control of volitional movements, we have carried out the following bi-directional axonal tract tracing experiments. Cats were initially deeply sedated via subcutaneous injection of medetomidine hydrochloride (Domitor) and subsequently anaesthetised via continuous intravenous infusion of propofol at a rate sufficient to abolish flexion reflexes. Fluorescent-labelled axonal transport tracing agents were micro-injected into the folia of Crus I to investigate spatial localization in the olivo-cerebellar and cortico-nuclear projections. The folia were shown to be mainly occupied in rostro-caudal succession by three zones receiving their olivo-cerebellar afferents from territories respectively confined within the dorsal lamella of PO, the ventral lamella and the rostral half of the medial accessory olive. These zones are presumably parts of the D2, D1 and C2 cerebellar cortical zones, as earlier proposed by Rosina & Provini (1982); they were respectively c. 1.5, 2.5 and 4.5 mm wide and their respective nuclear targets were in the rostro-ventral quadrant of NL, the caudo-ventral quadrant of NL and the ventral half of nucleus interpositus posterior. In the most medial one or two folia a narrow C3 zone between the C2 and D1 zones was related to the rostral part of the dorsal accessory olive and the dorso-caudo-lateral part of nucleus interpositus anterior. In the C2 and the D zones medial and lateral parts of the zone are innervated by different groups of olive cells and project respectively to medial and lateral parts of the nuclear territory for the zone, consistent with the existence of olivo-cortico-nuclear micro-complexes in Crus I (Ito, 1984). Within each zone a ‘folial’ localization is also present in both the olive and the cerebellar nuclei and intersects orthogonally with that for zone width.