Fossil chamaeleonid remains are usually restricted to isolated cranial bones, mainly jaw elements (Estes, 1983). The notable exception is the type and only known specimen of Chamaeleo intermedius, a natural calcite cast from the Middle Miocene of Fort Ternan, Kenya (Hillenius, 1978), which, on the basis of the contours of its head, clearly belongs in the genus Chamaeleo. We give here a preliminary description of a fossil skull from the Early Miocene (18 million years old) of Rusinga Island, Lake Victoria, Kenya. The original fossil is in the collections of the Kenya National Museums under the accession number KNM-RU 18340 (field number RU 1916'87). This preliminary description is based on a cast and on photographs of the original. The specimen was collected by one of us (AW) from bone lag resting on a weathering calcrete at site R107 in the Hiwegi Formation on Rusinga Island, Kenya. This site is approximately 34?9'0E, 0?25'30N and is recorded on Air Photo #215 in the Department of Palaeontology, National Museums of Kenya. The fossiliferous sediments on Rusinga have been shown to have been deposited during an interval of no more than half a million years at about 18 myr ago (Drake et al., 1988). These sediments are volcanoclastic tuffs, silts, sands and grits with occasional paleosols (Retallack, 1990). The R107 sediments have been hypothesized to have been deposited on a dry floodplain in the Rangwa volcanic arena (Pickford, 1986a). Other vertebrate and invertebrate species recorded from this site have been compiled by Pickford (1986b). The chamaeleonid skull is remarkably complete and undistorted, but as yet not fully prepared (Fig. 1). The distance from the anterior tip of the jaw symphysis to the posterior tip of the retroarticular process is 26.2