The only Dipterous parasites of tsetse-flies at present known belong to the family Bombyliidae, and to the genus Thyridanthrax, O. Sack., as enlarged by the late Professor Bezzi. Hitherto the species of this genus definitely stated in literature to be parasitic in the larval stage in Glossina pupae consisted solely of Th. (Villa) lloydi, Austen, and Th. (Anthrax) abruptus, Lw. In the following pages the number of species of which specimens have been bred from pupae of tsetse-flies is raised to seven, including three species which are described below for the first time. With one exception (Th. argentifrons, sp. n.), in which the holotype emerged from a pupa of Glossina tachinoides, Westw., while a paratype was bred from a pupa of G. morsitans, Westw., form submorsitans, Newst., these flies are not known to attack any species of Glossina other than G. morsitans. Whether it will ever prove possible to make practical use of parasites for the purpose of tsetse-fly control, is a question for the future. In this connection, however, it is perhaps of interest to note that, whereas six out of the seven species dealt with below occur either in Rhodesia, or in countries to the south-east or north-east of that territory, the seventh species (Th. argentifrons) has been met with in Northern Nigeria.