Eptesicus isabellinus was originally described as Vespertilio isabellinus by Temminck (1840, p. 205, pl. 52 figs. 1, 2) from a series of specimens obtained by J. F. H. Clifford Cocq van Breugel in the vicinity of Tripoli, Libya. Unfortunately Temminck's original description did not include any cranial measurements and as a result the precise affinities of this bat have been uncertain ever since. Lataste (1887, p. 625) described a small serotine bat from the vicinity of Cairo, which he named Vesperugo (Vesperus) Innesi. He stated that he had compared this with a specimen of E. isabellinus (the essential data of which were not given) and that E. innesi was smaller. Lataste evidently regarded E. isabellinus as a form of E. serotinus (Schreber, 1774). Once again, however, full cranial measurements were not given for the new form E. innesi. Harrison (1962) re-examined the lectotype and other specimens of E. innesi in the British Museum; extracted the skull of the lectotype and gave full cranial and external measurements. Setzer (1957, p. 40) in his review of Libyan mammals, treated E. isabellinus as a distinct species and stated that it had not been taken since Temminck's description. He noted that Zavattari (1934, p. 887) listed it as Vespertilio serotinus isabellinus. Allen (1939, p. 87) also listed it as a subspecies of E. serotinus. Ellerman & Morrison-Scott (1951, p. 156) treated E. isabellinus as a distinct species on the grounds that the measurements given by Tate (1942, p. 275) were too small for E. serotinus and they regarded Lataste's small species E. innesi as a subspecies of it. In view of all this uncertainty it seemed essential to re-examine the original type material of E. isabellinus and make direct comparisons with E. innesi and the European E. serotinus in order to finally clarify its status. Three