It has been found that the size of adults of the parasites Mutilla glossinae Turner and Thyridanthrax abruptus (Lw.) reared from puparia of Glossina differ with the size of the host, the puparia of the larger tsetse species, G. pallidipes Aust., producing larger parasite than the puparia of the smaller species, G. morsitans Westw. A significant relationship has been found between size of puparium (measured by puparial width) of G. morsitans and size of the parasite (measured by thoracic area and dry weight) for males of T. abruptus and males and females of M. glossinae, and it is suggested that this relationship may extend also to other species of Thyridanthrax and Mutilla that parasitise Glossina. Evidence, both from other workers and this laboratory, indicates that it is the different quantities of food supply in the hosts that affects the size of the adult parasite.