The lio gene encodes a putative receptor tyrosine kinase, with unique motifs both in the extracellular and catalytic domains (Dura, J.-M., Préat, T., Tully, T., 1993. Identification of linotte, a new gene affecting learning and memory in Drosophila melanogaster. J. Neurogenet. 9, 1-14). We show here that a complete deletion of lio activity causes specific structural defects in the adult brain. Gal4 enhancer-trap lines used as cell markers revealed that in lio mutants central brain axons behave as if they were abnormally attracted by the midbrain area. The Lio protein is expressed in third instar larvae in a few cells at the junction of the cerebral hemispheres. These glial cells form a newly described ring structure, showing an invariable fibrous organization. In the wild-type this ring disappears at midpupation. Our results indicate that the Lio putative kinase plays a major role in the modeling of the adult brain by controlling the fate of the transient interhemispheric ring.