Abstract.Midgut protease activity inGlossina morsitans centralisandG. m. morsitans, at 48h post bloodmeal averaged 1.8IU of trypsin‐like activity. These two tsetse subspecies differ in their susceptibility to trypanosome infection. Except for low levels in flies fed on waterbuck blood (0.7IU), activity did not differ in flies fed a variety of host bloods (goat, pig, cow, buffalo, eland) and trypanosome species (Trypanosoma congolense, T.brucei, T.simiae). Protease activity was also not correlated with infection rates, despite large differences in infection rates among experiments. Nevertheless, addition of 0.06M D(+)‐glucosamine to parasitaemic blood resulted in a three‐fold reduction in protease activity, coincident with a large increase in infection rate. This effect did not occur when parasites or D(+)‐glucosamine were added alone to the bloodmeal, suggesting that the effect was due to metabolism of D(+)‐glucosamine by