The mode of neuron migration from the site of their origin in the ventricular zone to area CA1 of the hippocampus was analysed with Golgi and electron microscopic methods during the first half of gestation in the foetal rhesus monkey. In the inner portion of the intermediate zone, the migrating cells have a bipolar form with one, or oaccasionally two, leading processes which do not reach the ammonic plate and with a single trailing process which usually ends within the intermediate zone. Both the nucleus and the cytoplasm of the migrating cells are relatively electron-dense and the latter contains organelles typical of young neurons as described in other brain regions. Analysis of electron micrographs from serial sections reveals that the length of the somata and of the leading and trailing processes of the migrating neurons is apposed to fascicles of radially oriented, electron-lucent, microtubule-filled fibres which are ultrastructurally similar to the radial glial fibres of the neocortex and to the Bergmann glial fibres of the cerebellum. The close (20 nm) apposition between the membranes of the migrating cell and the radial fibre is maintained even in areas where the fibres bend or curve tortuously. Migrating neurons situated at progressively more superficial levels of the intermediate zone become progressively more differentiated and complex. Thus, in the outer portion of the intermediate zone, the migrating cells acquire several additional cytoplasmic processes and occasionally a long thin axon-like process which courses into the incipient alveus. These cells have somewhat larger somata and less electron-dense nuclei and cytoplasm than the migrating neurons still situated in the inner part of the intermediate zone. Cells close to the ammonic plate usually have one to three cytoplasmic processes that enter the ammonic plate and terminate near their presumed final position. Migrating neurons situated at the lower border of the ammonic plate have a single large apical process which intermingles with neurons already in their final position and which sometimes traverses the ammonic plate. The apposition of the migrating neurons to the radial glial processes becomes less explicit as the cell soma enters the ammonic plate, reflecting the more complex three-dimensional intercellular relationships. However, the present analysis indicates that during the middle and late stages of neuronal migration to the hippocampus radial glial fibres may guide postmitotic young neurons across the intermediate zone to the ammonic plate in the same way that they guide neurons migrating to the superficial and middle layers of the neocortical plate.