The pathogenicity of cyclically transmitted C16 clone 1 of Trypanosoma brucei metacyclics which developed within normal and hypertrophied DNA virus-infected salivary glands of Glossina morsitans morsitans was studied in the inbred, BALB/c mice. Microscopic examination of salivary glands obtained from G. m. morsitans with combined virus and T. brucei infections (i.e. the flies which transmitted the trypanosome infection to the test group of mice, group A) revealed that the glands comprised hyperplastic epithelial cells, some of which were fragmented, and numerous metacyclic trypanosomes. All the mice which contracted trypanosomiasis from G. m. morsitans with only T. brucei, i. e., the control mice (Group B; n=4) and the mice in the test group (n = 3) developed high parasitaemia and died. Repeated-measures analysis of variance using adjusted mean rate revealed that the mean prepatent periods and the mean times to death in both the control and the test groups of the BALB/c mice were not significantly different (P > 0.05). Analysis of log-transformed data fitted to a logistic growth model revealed that the rate of multiplication of T. brucei parasites in the blood of the test group BALB/c mice (r=2.373) was greater than in the control mice (r=0.808) and that the maximum carrying capacity was also attained earlier in the former group. These observations imply that the development of T. brucei metacyclics within hypertrophied salivary glands and their co-existence with the DNA virus particles might have enhanced their infectivity and virulence in the mice.